[Roverlord]

I've had .dev domains for years. (Internally with Apache, virtual hosts IIRC?)

It seemed like the obvious choice of suffix for my localhost experiments, and I can't be the only person using it (e.g. https://headsigned.com/posts/setting-up-local-development-do...), will this new TLD break that?

I suppose I can always edit my hosts file but if I didn't know a new TLD was dropping would it screw me silently?

[PinguTS]

You should never ever invent your own TLD for internal use. There TLD defined for that purpose. Look at RFC2606.

[RFC2606] https://tools.ietf.org/html/rfc2606