It is nice to support everyone but with all the versions that the “MUI for AmigaOS” people released they made it a mess for all users and developers on all platforms. The only useful solution is to support (only) MUI 3.8 for OS3.x and use MUI5 on MOS (maybe on OS3/4 as well if available, but I read of several incompatibilities on OS3/4). If you read OS4 forums, many users went back to use MUI 3.8 for two reasons: (1) they did not have any benefit from higher versions or (2) the higher versions are too slow for them. And the few people who use higher than 3.8 versions: who should use MUI4 if MUI5 is available for free?