Internal. is also an valid option. I moved everything there about a year ago

.internal is ICANN and not (yet?) an IETF RFC:

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

Other special use domains:

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

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

* https://datatracker.ietf.org/doc/html/rfc6761