I love Warsow but am I right that it's very hard to find an opponent these days? I just checked https://arena.sh/wa/ and there are 4 non-empty servers, but most (maybe all) of the players seem to be bots.

I haven't played a while so I cannot comment. When I last played I spun up a server for my friend group to play on. This is the beauty of old school games like these, no need to rely on a company to keep servers running. In a way Warsow is the perfect LAN game: everyone can run it and it's easy to host a server.