To this date, I was unaware of the many emojis offered on #Mastodon servers. Shame on me, as it might be a real instance selection criterion. Find out what any instance supports here: https://emojos.in/ 
@tyrandus Most platforms offer custom emojis, actually. It's not Mastodon-specific.
@trinsec Enabled by Mastodon, though. 
@tyrandus That wasn't the point you raised originally, though. It's just that many platforms offer it and if you wanted one to be added, you probably can ask your admin for that. Some are very accomodating.