DEPR: Deprecate is_copy (#18801)
Not all tests for this branch on the fork are passing, but these are the same tests that fail for the clean master and appear to be unrelated to the changes.

References to is_copy in tests were changed to the internal attribute _is_copy assuming that, though is_copy is not meant to be a public attribute, checking the state of this attribute is necessary to check consistent internal behavior.

Closes #18801.