Did setting SF_FIRMLINK ever succeed for real? Firmlinks are an APFS addition, and AFAIU not only can't usercode directly create firmlinks, trying to set SF_FIRMLINK with chflags is non-sensical, sort like trying to use chmod to set the S_IFDIR bit on a regular file. Apple would hardly be the first OS to silently ignore non-sensical flags in a bitfield; such problems and their dilemmas have plagued Linux syscalls for years. Though if this is a change of behavior--it previously always failed but now it doesn't--that's problematic. Alternatively, if previously chflags seemed to work--st_flags reflected the SF_FIRMLINK bit on subsequent stat, even if the inode wasn't actually a firmlink--the change would be closer to a fix than a regression. I'm more curious why you were trying to set it in the first place. Maybe I'm misunderstanding.