Checking the class_exists for Wikibase which would complicate enabling preload (feature of php 7.4) can be found everywhere in Wikibase-related extensions: the whole list. For example: In WikidataPageBanner. They are there because Wikibase wasn't being loaded using extension registry (but that got fixed recently) and now they can simply migrated to ExtensionRegistry::isLoaded('WikibaseClient') and ExtensionRegistry::isLoaded('WikibaseRepo').
List of related ones already found:
- MediaWiki Extensions no longer use class_exists checks for Wikibase classes