Currently SDL3 is being installed the hard way (git clone; make; sudo make install), we need to come up with a plan for this to change. Possible alternatives:
vendor sdl inside the project statically
vendor sdl inside the project, giving a dynamic library
wait for sdl to be available in the next ubuntu LTS, which seems to be the most common denominator for old distros. (probably going to take 2 years or more)
vendor SDL3 statically only for CI builds, but demand that developers that compile the code handle the problem of providing SDL3 themselves.
SDL3 is not release yet, but has hit ABI freeze. Until it's officially released this PR should not be merged.