https://www.google.com/search?q=apple%20eu%20alternative%20a...
The end result of which, if done at large scale, means that an EU government couldn't ban signal, short of forcing all its domestic ISPs to be downstream of a China type great firewall, or simply null route all the IP space where signal's servers are located.
Side-loading is harder to enforce any rules over, of course.
Blocking domains is well-established at this point, thanks to the copyright industry doing a 21.5-year whack-a-mole-waltz with The Pirate Bay. Of course, this also demonstrates the limited effectiveness of domain blocking.
Extremely limited effectiveness, when VPN operators like Mullvad are corporations based in Sweden and offer 5 euro a month service to bypass whatever local "mess with internet traffic" activity, whether government-caused or not, that someone's last mile ISP is up to...
There's also the game of whack a mole with taking ownership of domains at the registrar/ICANN level through court orders, such as with the various .com or similar things that get jacked and plastered with a "DOMAIN HAS BEEN SEIZED" notice by the US feds.