Problem/Motivation
This issue is spun off from #3332593: Adopt PluginExists validator in relevant places.
In that issue, we added validation for the field.field_settings.entity_reference:handler property in config schema, validating that the entity reference selection handler does, in fact, exist.
To manage scope and prevent potential backwards compatibility breaks, we are currently allowing the fallback plugin ID (broken) to be stored in config. But that's not really okay - config should never be referring directly to a fallback plugin, except in very limited circumstances (like block_content:UUID plugins, which might refer to a block content entity that doesn't exist).
Proposed resolution
Remove the allowFallback: true line from field.field_settings.entity_reference:handler's PluginExists constraint. Then fix any tests that break because of that, and create an update path if necessary.
User interface changes
Shouldn't be any.
API changes
Unclear, but there shouldn't be.
Data model changes
Possible; TBD.
Release notes snippet
TBD