Nothing stops them from hosting it on fbi.gov, state.gov, etc.

It's one thing to block some random .gov site unused for anything else, it's another thing to block a domain used for, say, filing flight plans.

Europeans don't generally use .gov so if the US tries to pull that, they'll just block whatever .gov their VPN is hosted on.

Southern European countries are blocking whole Cloudflare IP ranges because of the massive grip on the government the sports licensing maffia has there. These countries also don't feature any direct flights to America as far as I can tell.

These blocks may cause (temporary) issues for American business relations and tourism, but such side effects may not be considered so problematic if the US leverages their government infrastructure to attack European legislators.

Nit: If you're filing a flight plan, you do it with the country you're departing from. Even if you're piloting an aircraft departing into the US, it wouldn't have any effect on operations if you couldn't reach US websites. There's also several alternative ways for pilots to file flight plans outside of the web.

(The flight plans get passed between countries via AFTN/AMHS, which are dedicated telecommunications networks independent of the Internet.)

If a Govt decides that I am pretty sure they won't stop at anything but TLD level banning. Besides I don't know about other countries (or EU) but I won't be surprised if our giant industrious neighbour already has infrastructure in place just for such Trumpian shenanigans :)