[quazeekotl]

BSDs also have a command that does the same thing, but its called chflags

https://man.openbsd.org/chflags.1

https://www.freebsd.org/cgi/man.cgi?query=chflags

http://netbsd.gw.com/cgi-bin/man-cgi?chflags

[jwilk]

AFIACS, chattr(1) can be compiled on BSDs too; and then it uses the chflags(2) syscall.