It may be technically difficult / expensive / annoying if it was not designed into their system. Your linux box doesn't has hordes of peers / backup mechanisms and cross references to users, to a point that it may be impractical or not cost effective to conclusively prove that a user is deleted everywhere

Deletion may not have been in the original spec or in any of the specs that were bolted on.

Sure, ok. But whatever happens is still an internal process. Not a legal one.