VTK doesn’t really “do” moves, but it probably isn’t safe (e.g., things the ctor constructs may not be reconstructible via methods to “restore” such a member to a valid state). I forget the rules of move operators and ctors…they might not be made at all given the set of other magic methods in VTK classes.
As for the operators, the return type can’t be overloaded, so it doesn’t really matter what it returns. There are a number of (suppressed) clang-tidy warnings around these methods anyways:
the return type as you suggest
that they are private (users first get the error that they are inaccessible, not that they are explicitly deleted)