On the Fireside Fedi interview with Jerry ( the admin of Infosec.Exchange Mastodon instance ) a scary truth was suddenly revealed ( on 34:11 ): Just to keep the instance up and running he needs to spend up to $5000 a month, pretty much out of his pocket. Donations to the instance barely cover any of that. And if he will ask people to pay to use it, they will, rightfully so, switch to a different instance.
Misskey is probably the only fediverse software that actually allows admin instance to put ads.
Its flagship instance, misskey.io (which also the second/third (?) biggest instances on fediverse), use freemium scheme for running the server. They have to do this as they have 600K users, with 20K visits per day. Their paid tier upgrades are mostly adding non-essentials stuff, such as drive capacity from 5GB to 30-100GB, profile and avatar decoration (similar to Discord stuff), or more webhook. They runs community ads, from indie games, vtuber promotion, comic release, or local art event. They also have one corporate backer, Skeb.jp, which an art commissioning platform.
Not saying that all instance should do this, but it could be a great learning.
I wouldn’t mind ads like these.