I assume !newcommunities@lemmy.world is for just that, and promoting new instances is unacceptable there. Where is an appropriate place to advertise a new instance?
Asking for one that is not my own. Decentralization is cool and I literally just found out about the existence of https://fanaticus.social/, a sports instance, from checking the !olympics@sh.itjust.works sidebar. I'd like to help this sports instance get attention and bolster its existing communities. I don't know where to go besides finding big preexisting sports communities and mentioning its existence there.
There's a lot. I do not want to spam !newcommunities@lemmy.world, and they're also not new, although I know not-new-but-still-small communities have been allowed to get posted before. Now that you mention it, I could probably just make a few posts or one giant post listing related communities together…
!communitypromo@lemmy.ca is also intended for this purpose. You can promote any community or instance at any point, as long as you don't promote it too often :)