sdl3-client
SDL3 should come out quite soon. Because of that, I think we should work toward it in 3.2 already. If the sdl3-client does not really make it to freeciv-3.2, it's easy to drop, but it would be hard to backport it from 3.3 (main) at the last moment.
For now, this ticket is about following what happens on libSDL upstream, conserning SDL3: https://discourse.libsdl.org/t/sdl-3-0/40374