I have never accessed any of my servers from the internet and haven’t even adjusted my router firewall settings to allow this. I kept wanting to but never got around to it.
Does that mean you realistically don’t even know your network (router) setup? Because it’s entirely possible your machine is completely open to the internet - say, thanks to IPv6 autoconfiguration - and you wouldn’t even know about it.
It’s pretty unlikely but could potentially happen with some ISPs. Please always set up a firewall, especially for a server type machine. It’s really simple to block incoming outside traffic.