It appears Mongoid 7.5 legacy_attributes is in the release notes, but not in the actual code. Perhaps something was missed in the backport.

https://www.mongodb.com/docs/mongoid/current/release-notes/mongoid-7.5/#force-the-attributes-method-to-always-return-a-hash

I imagine there should be a 7.5.1 release since this is meant to be a deprecation change, and Mongoid 8.0 sets the value to "false".