First of all I think this is generally a good move. If people use random TLDs for testing then that’s just bad practice and should be considered broken anyway.
But second I think using local host names should be considered a bad practice anyway, whether it’s reserved names like .test or arbitrary ones like .dev. The reason is that you can’t get valid certificates for such domains. This has caused countless instances where people disable certificate validation for test code and then ship that code in production. Alternatively you can have a local CA and ship their root on your test systems, but that’s messy and complicated.
Instead best practice IMHO is to have domains like bla.testing.example.com (where example.com is one of your normal domains) and get valid certificates for it. (In the case where you don’t want to expose your local hostnames you can also use bla-testing.example.com and get a wildcard cert for *.example.com. Although ultimately I’d say you just shouldn’t consider hostnames to be a secret.)