hook_field_attach_form was apparently lost in the battle in #2095195: Remove deprecated field_attach_form_*() → . Ow as @tim.plunkett put it in #2095195-108: Remove deprecated field_attach_form_*() → :
Ouch, I missed that this removed hook_field_attach_form(). That will hurt, as it was a nice pairing for hook_field_extra_fields() in D7. I honestly have no idea how to modernize that code now, and the change notice (https://drupal.org/node/2225801) is absolutely zero help. It just says "Use the Entity API instead", as though that will impart some information.
In fact there is currently no sane way to add extra fields to entity forms. And maybe as of this ds_forms did not get ported, and surely as of this we are stuck in EVA's #3031564: Attach View to Entity Form (mode) → .
Bug as of regression, major as of contrib bollocker.
Re-add the hook.
TBD
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
#2095195: Remove deprecated field_attach_form_*() → had a todo about a new hook:
+++ b/core/modules/node/node.api.php
@@ -91,7 +89,7 @@
- * - field_attach_form()
+ * - @todo hook for EntityFormDisplay::buildForm()
That todo got lost in #2216535: Replace Node overview topic and Node API topic with Entity Hooks topic → 8 commits later.