Context https://github.com/rails/rails/pull/39599#discussion_r445429365.

Similar with #39611.

Actually we never said we would keep Mashal compatibility between two
versions of Rails, so we sometimes directly removed the dedicated types
in the past (e.g. f1a0fa9e, #29666), but that direct removing makes cache
rotation harder.

Keeping the unused constant in new version at least during one version
will help cache rotation easier.