The PageEditStash uses PHP serialization of ParserOutput, despite that having been deprecated in 1.41 and support removed in 1.45 (T353570).
The alternative JSON serialization should probably be backported as far back as 1.43 as well to allow some measure of rollback compatibility.
It also uses PHP serialization of the Content object; that should have a supported (and tested) JSON serialization with a compatibility policy as well.
T264389: ProofreadPageContent must not contain User object, since it cannot be serialized safely. discusses some of the existing issues with Content objects and PHP serialization.