>I read manpage on github, but did not understood, what exactly this > program provides. Can it replace creation system users for dropping > privileges?
It's doesn't create users. It only drops privileges (extremesetuidgid) or sets $UID/$GID env. variables (extremeenvuidgid). For example: extremesetuidgid -b 100000 sleep 1 runs command 'sleep 1' under unprivileged uid/gid (computed getpid() +100000) e.g. for: pid=10 ... uid=gid=100010 pid=11 ... uid=gid=100011 pid=12 ... uid=gid=100011 ...
