i like it when members can add their own emojis! slack does show you /who/ added one so you could reach out to someone if they add something inappropriate. i think a notif via tightknit hub would be helpful in terms of monitoring that. i love an inside joke slack emoji / react!! and it's fun to add the usual party parrots or lolsobs or shocked pikachu.
my caveats here are: i've only managed smaller (sub 2k) slack communities. i think if i were managing like a 5k+ one, i might not allow it. but also depends on the nature of the group!! slack communities tend to be more professional driven and most people are there with their real names, tied to their careers, so there's often more maturity (in comparison to something like discord)