.internal is reserved by ICANN, but there is no IETF RFC ('just' a draft (for now)):

* https://en.wikipedia.org/wiki/Top-level_domain#Reserved_doma...

* https://en.wikipedia.org/wiki/Special-use_domain_name

* https://en.wikipedia.org/wiki/.internal

* https://datatracker.ietf.org/doc/html/draft-davies-internal-...

Not sure how important that is.