The gcc-15 uses by default the C23 standard which dropped the K&R syntax support which procmail heavily uses. So that's why we used the -std=gnu89 in Fedora upstream. I think that's the K&R is too old, that today it doesn't make sense to use it for backward compatibility. If there is no other reason for it, so maybe it could be dropped. It would be a lot of changes so I would like to hear the new upstream opinion about it. If you are for it, I could try to PR something.