It's complicated.

In most of the US except for a handful of states, you can legally own a silencer, there's a couple extra hoops to jump through, but nothing too dramatic. Most law abiding gun owners in the US could probably get their hands on one legally if they cared enough to do so.

In other parts of the world, laws are kind of all over the place ranging from totally unregulated to totally illegal. I haven't done a deep dive on this myself, so this is all 2nd or 3rd+ hand hearsay so take it for what it's worth, but I'm told that some places regard them as almost a safety (hearing protection) or courtesy thing so you're not making too much racket with your guns.