It was asked a couple of months ago, but given the AI enshittification, it seems a good time to ask again.
I looked around and found:
- Pinetta is/was in development but seems dead with only an alpha release. I am chasing this up on Matrix. edit: Apparently still, technically, active although progress has been slow
- Pinry is a FOSS alternative, but not federated, that seems dormant. However, it works and is in Python, so someone could bolt an AP server to it, he says airily.
Open source serving algorithms exist. They have the benefit of being customizable, not purely engagement driven.
As for tagging, I think Ao3’s system is exemplary: https://fanlore.org/wiki/AO3_Tagging_System
That sounds good! If Pixelfed or similar implemented that, it could be sufficient. A new fedi service starting from scratch would lack the existing pool of resources since federation only happens when a post gets posted/boosted but after some time it could actually be useful.