Problem/Motivation
If you add custom block to page, then remove the block itself but don't touch the node, you'll get "This block is broken or missing. You may be missing content or you might need to enable the original module." rendered instead of the block. This is good, but in logs you'll get notices. If these are also shown in UI (PHP notices enabled from settings.php), then these can also break the site because of "ailed to start the session because headers have already been sent by ..."
Steps to reproduce
Go to https://simplytest.me/, start Gutenberg 2.6.
Enable Gutenberg on one the content types.
Create new node, edit with source and add:
<!-- wp:drupalblock/block-content-e5ac6348-b1be-460e-b1ac-25c5966d21df {"blockId":"block_content:e5ac6348-b1be-460e-b1ac-25c5966d21df"} /-->
View the node. You'll see: "This block is broken or missing. You may be missing content or you might need to enable the original module."
Now check logs, you'll see the multiple notices:
Notice: Undefined index: provider in user_preprocess_block() (line 341 of /var/lib/tugboat/stm/web/core/modules/user/user.module)
#0 /var/lib/tugboat/stm/web/core/includes/bootstrap.inc(347): _drupal_error_handler_real(8, 'Undefined index...', '/var/lib/tugboa...', 341)
#1 /var/lib/tugboat/stm/web/core/modules/user/user.module(341): _drupal_error_handler(8, 'Undefined index...', '/var/lib/tugboa...', 341, Array)
#2 [internal function]: user_preprocess_block(Array, 'block', Array)
#3 /var/lib/tugboat/stm/web/core/lib/Drupal/Core/Theme/ThemeManager.php(287): call_user_func_array('user_preprocess...', Array)
#4 /var/lib/tugboat/stm/web/core/lib/Drupal/Core/Render/Renderer.php(433): Drupal\Core\Theme\ThemeManager->render('block', Array)
#5 /var/lib/tugboat/stm/web/core/lib/Drupal/Core/Render/Renderer.php(446): Drupal\Core\Render\Renderer->doRender(Array)
#6 /var/lib/tugboat/stm/web/core/lib/Drupal/Core/Render/Renderer.php(204): Drupal\Core\Render\Renderer->doRender(Array, false)
Notice: Undefined index: provider in shortcut_preprocess_block() (line 241 of /var/lib/tugboat/stm/web/core/modules/shortcut/shortcut.module)
#0 /var/lib/tugboat/stm/web/core/includes/bootstrap.inc(347): _drupal_error_handler_real(8, 'Undefined index...', '/var/lib/tugboa...', 241)
#1 /var/lib/tugboat/stm/web/core/modules/shortcut/shortcut.module(241): _drupal_error_handler(8, 'Undefined index...', '/var/lib/tugboa...', 241, Array)
#2 [internal function]: shortcut_preprocess_block(Array, 'block', Array)
#3 /var/lib/tugboat/stm/web/core/lib/Drupal/Core/Theme/ThemeManager.php(287): call_user_func_array('shortcut_prepro...', Array)
#4 /var/lib/tugboat/stm/web/core/lib/Drupal/Core/Render/Renderer.php(433): Drupal\Core\Theme\ThemeManager->render('block', Array)
#5 /var/lib/tugboat/stm/web/core/lib/Drupal/Core/Render/Renderer.php(446): Drupal\Core\Render\Renderer->doRender(Array)
#6 /var/lib/tugboat/stm/web/core/lib/Drupal/Core/Render/Renderer.php(204): Drupal\Core\Render\Renderer->doRender(Array, false)
There are more.
In our own site, we are running PHP 8.1, Drupal 9.4.10
Proposed resolution
None of the notices should be generated.