Correctly disabling the functionality and "uninstalling" Field Default Token the correct way will still resolve in WSOD with the following Error message on screen.
TypeError: call_user_func(): Argument #1 ($callback) must be a valid callback, function "field_default_token_default_value_callback" not found or invalid function name in call_user_func() (line 400 of /var/www/html/dp/web/core/lib/Drupal/Core/Field/FieldConfigBase.php)
Additional problem is that this error occurs not immediately when caching is active, and the web administrator will be surprised about this White Screen Error maybe days later on particular parts of the site where the functionality oft his module was involved before. It seems that correctly disabling and uninstalling the module still lefts function calls in the database, what, for me, looks like a not properly coded uninstalling process for this project, IMHO.
The full Error message from the logs is:
TypeError: call_user_func(): Argument #1 ($callback) must be a valid callback, function "field_default_token_default_value_callback" not found or invalid function name in call_user_func() (line 400 of /var/www/html/dp/web/core/lib/Drupal/Core/Field/FieldConfigBase.php)
#0 /var/www/html/dp/web/core/lib/Drupal/Core/Field/FieldConfigBase.php(400): call_user_func()
#1 /var/www/html/dp/web/modules/contrib/viewfield/src/Plugin/Field/FieldFormatter/ViewfieldFormatterDefault.php(116): Drupal\Core\Field\FieldConfigBase->getDefaultValue()
#2 /var/www/html/dp/web/core/lib/Drupal/Core/Field/FormatterBase.php(89): Drupal\viewfield\Plugin\Field\FieldFormatter\ViewfieldFormatterDefault->viewElements()
#3 /var/www/html/dp/web/modules/contrib/viewfield/src/Plugin/Field/FieldFormatter/ViewfieldFormatterDefault.php(95): Drupal\Core\Field\FormatterBase->view()
#4 /var/www/html/dp/web/core/lib/Drupal/Core/Entity/Entity/EntityViewDisplay.php(265): Drupal\viewfield\Plugin\Field\FieldFormatter\ViewfieldFormatterDefault->view()
#5 /var/www/html/dp/web/core/modules/layout_builder/src/Entity/LayoutBuilderEntityViewDisplay.php(266): Drupal\Core\Entity\Entity\EntityViewDisplay->buildMultiple()
#6 /var/www/html/dp/web/core/lib/Drupal/Core/Entity/EntityViewBuilder.php(340): Drupal\layout_builder\Entity\LayoutBuilderEntityViewDisplay->buildMultiple()
#7 /var/www/html/dp/web/core/modules/node/src/NodeViewBuilder.php(24): Drupal\Core\Entity\EntityViewBuilder->buildComponents()
#8 /var/www/html/dp/web/core/lib/Drupal/Core/Entity/EntityViewBuilder.php(282): Drupal\node\NodeViewBuilder->buildComponents()
#9 /var/www/html/dp/web/core/lib/Drupal/Core/Entity/EntityViewBuilder.php(239): Drupal\Core\Entity\EntityViewBuilder->buildMultiple()
#10 [internal function]: Drupal\Core\Entity\EntityViewBuilder->build()
#11 /var/www/html/dp/web/core/lib/Drupal/Core/Security/DoTrustedCallbackTrait.php(101): call_user_func_array()
#12 /var/www/html/dp/web/core/lib/Drupal/Core/Render/Renderer.php(788): Drupal\Core\Render\Renderer->doTrustedCallback()
#13 /var/www/html/dp/web/core/lib/Drupal/Core/Render/Renderer.php(374): Drupal\Core\Render\Renderer->doCallback()
#14 /var/www/html/dp/web/core/lib/Drupal/Core/Render/Renderer.php(204): Drupal\Core\Render\Renderer->doRender()
#15 /var/www/html/dp/web/core/lib/Drupal/Core/Template/TwigExtension.php(479): Drupal\Core\Render\Renderer->render()
#16 /var/www/html/dp/web/sites/default/files/php/twig/63c722de11506_block.html.twig_B8ibzOM-ZeAYyf9ED3OxZ9irM/bEa0tJEcktqkOhm_UbAtacXPoan0bQR8ipcrmU7ozsg.php(80): Drupal\Core\Template\TwigExtension->escapeFilter()
#17 /var/www/html/dp/vendor/twig/twig/src/Template.php(182): __TwigTemplate_21737e91eb318a9cc0edc14b7967a238->block_content()
#18 /var/www/html/dp/web/sites/default/files/php/twig/63c722de11506_block.html.twig_B8ibzOM-ZeAYyf9ED3OxZ9irM/bEa0tJEcktqkOhm_UbAtacXPoan0bQR8ipcrmU7ozsg.php(68): Twig\Template->displayBlock()
#19 /var/www/html/dp/vendor/twig/twig/src/Template.php(405): __TwigTemplate_21737e91eb318a9cc0edc14b7967a238->doDisplay()
#20 /var/www/html/dp/vendor/twig/twig/src/Template.php(378): Twig\Template->displayWithErrorHandling()
#21 /var/www/html/dp/vendor/twig/twig/src/Template.php(390): Twig\Template->display()
#22 /var/www/html/dp/web/core/themes/engines/twig/twig.engine(55): Twig\Template->render()
#23 /var/www/html/dp/web/core/lib/Drupal/Core/Theme/ThemeManager.php(384): twig_render_template()
#24 /var/www/html/dp/web/core/lib/Drupal/Core/Render/Renderer.php(433): Drupal\Core\Theme\ThemeManager->render()
#25 /var/www/html/dp/web/core/lib/Drupal/Core/Render/Renderer.php(446): Drupal\Core\Render\Renderer->doRender()
#26 /var/www/html/dp/web/core/lib/Drupal/Core/Render/Renderer.php(446): Drupal\Core\Render\Renderer->doRender()
#27 /var/www/html/dp/web/core/lib/Drupal/Core/Render/Renderer.php(204): Drupal\Core\Render\Renderer->doRender()
#28 /var/www/html/dp/web/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php(242): Drupal\Core\Render\Renderer->render()
#29 /var/www/html/dp/web/core/lib/Drupal/Core/Render/Renderer.php(580): Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}()
#30 /var/www/html/dp/web/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php(243): Drupal\Core\Render\Renderer->executeInRenderContext()
#31 /var/www/html/dp/web/core/lib/Drupal/Core/Render/MainContent/HtmlRenderer.php(132): Drupal\Core\Render\MainContent\HtmlRenderer->prepare()
#32 /var/www/html/dp/web/core/lib/Drupal/Core/EventSubscriber/MainContentViewSubscriber.php(90): Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse()
#33 [internal function]: Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray()
#34 /var/www/html/dp/web/core/lib/Drupal/Component/EventDispatcher/ContainerAwareEventDispatcher.php(142): call_user_func()
#35 /var/www/html/dp/vendor/symfony/http-kernel/HttpKernel.php(174): Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch()
#36 /var/www/html/dp/vendor/symfony/http-kernel/HttpKernel.php(81): Symfony\Component\HttpKernel\HttpKernel->handleRaw()
#37 /var/www/html/dp/web/core/lib/Drupal/Core/StackMiddleware/Session.php(58): Symfony\Component\HttpKernel\HttpKernel->handle()
#38 /var/www/html/dp/web/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(48): Drupal\Core\StackMiddleware\Session->handle()
#39 /var/www/html/dp/web/core/modules/page_cache/src/StackMiddleware/PageCache.php(191): Drupal\Core\StackMiddleware\KernelPreHandle->handle()
#40 /var/www/html/dp/web/core/modules/page_cache/src/StackMiddleware/PageCache.php(128): Drupal\page_cache\StackMiddleware\PageCache->fetch()
#41 /var/www/html/dp/web/core/modules/page_cache/src/StackMiddleware/PageCache.php(82): Drupal\page_cache\StackMiddleware\PageCache->lookup()
#42 /var/www/html/dp/vendor/asm89/stack-cors/src/Asm89/Stack/Cors.php(49): Drupal\page_cache\StackMiddleware\PageCache->handle()
#43 /var/www/html/dp/web/modules/contrib/advban/src/AdvbanMiddleware.php(57): Asm89\Stack\Cors->handle()
#44 /var/www/html/dp/web/core/modules/ban/src/BanMiddleware.php(50): Drupal\advban\AdvbanMiddleware->handle()
#45 /var/www/html/dp/web/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(48): Drupal\ban\BanMiddleware->handle()
#46 /var/www/html/dp/web/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(51): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle()
#47 /var/www/html/dp/web/modules/contrib/remove_http_headers/src/StackMiddleware/RemoveHttpHeadersMiddleware.php(49): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle()
#48 /var/www/html/dp/vendor/stack/builder/src/Stack/StackedHttpKernel.php(23): Drupal\remove_http_headers\StackMiddleware\RemoveHttpHeadersMiddleware->handle()
#49 /var/www/html/dp/web/core/lib/Drupal/Core/DrupalKernel.php(713): Stack\StackedHttpKernel->handle()
#50 /var/www/html/dp/web/index.php(19): Drupal\Core\DrupalKernel->handle()
#51 {main}
It only came into my focus after a drush cache clear and cron run popping up in terminal just twice. But in fact it had already crashed many sub sites with content and fields which used to have settings from this module before.
From my point of view regarding discussed/documented issue flags this is a "Major" if not "Critical issue, rendering parts of the front side completely broken without warning and without any chance to repair this. These calls are spread over the whole database in any place where fields got settings setup from this module.
As this project is still in dev status for Drupal 8 and higher and should not be used on non testing enviroments, I leave it by "Major". But I stongly recommend not to install this project if you are not 100% sure about what you are doing and not before backing up the database completely. It's not just removing config.
Possible next steps to do to make this issue less harmful and to solve the underlying problem.
I tend to recommend steps from the quickest to the hardest/longest tasks in that order on issues like this. Especially on dev projects to make the project usable/testable again as fast as possible.
So first thing I would do is to put a warning on the project page (maintainers?) here on Drupal linking to this issue and stating that this module cannot be uninstalled properly at the moment until this issue is solved in D8 and higher.
Second thing is to maybe rethink about how to build proper uninstall routines for this project or maybe even consider another approach to provide the functionality of the project instead of as it is now. For me it seems a little bit clunky to have calls to undefined functions in the database when a module has been uninstalled. But this is something maintainers and users should discuss together before starting any wasting work on patches.