Signal doesn't collect IPs and therefore can't even hand them out. It's been requested in 2021. Here's a list of requests from authorities they were allowed to publish. I've looked through 3 of the most recent and nowhere do they reveal IPs.

Go ahead and create the group then make a post. Nobody's holding you back my friend.

What are the "popular" alternatives? Telegram stores everything, WhatsApp doesn't allow usernames, Matrix requires IPs too...

For fun and games in the cockpit?

Make it and share it. I don't have simplex.

Feel free to make a post about it. Very few will see it here otherwise :)

Same in a few other European countries. I'm doing it right now with a few SIMs. You can also go on holiday to another country, get a few temporary SIMs there for a few quid and fly back home with the "contraband". Really not hard.

Signal is a walled garden?

Then create a simplex group and invite everybody there.

Travel across the border where you can? Ask a friend abroad to send you a SIM card? Maybe even find a service to buy a SIM from?

Sometimes, but that's it. Authorities and signal itself can only say "this number has account with us since $signupDate and used it last on $usageDate". Signal can't say "we know $number is talking to $otherNumber" nor can they say "$number is in $group talking to $users".

Which ones do you suggest?

Feel free to create the chat rooms and share them as a post :)