I can only object to this claim, if thought through in detail. SDL 1.2 runs as well as ever on all versions of the PC operating systems Linux, Windows and OS X - there simply is no "more compatible" here and it is not like there are any other "modern operating systems" coming out for the PC in the near future. It is not like we have any major OS changes coming up the next years (like MS switching away from DOS or something...) which could possibly break what SDL does for quake3 (which is 1: mouse and keyboard input, 2: opening a window with an OpenGL context. Audio and 2D acceleration are done by OpenGL and OpenAL anyway.)
Correct me if I am wrong please.
Maybe there will be an SDL3 when this whole ChromeOS, FirefoxOS trend progresses any further. Maybe then a few years into the future SDL 1.2 support will be dropped. Maybe there will be a Windows 10 that is binary incompatible and a new processor architecture? But now it isn't like that and it won't be any time soon.