ios apps that allow custom feeds?

I don't know any, but Lemmy will be getting native support for multicommunities, thanks to NLnet.

https://join-lemmy.org/news/2024-09-11_-_New_NLnet_funding_for_Lemmy

Aside from being just a generally great Lemmy app, !arctic@lemmy.world does this.

There's an app called Loom that is supposed to do this as a major feature. It's still in an early dev stage though so still not super polished.