A quick, but a little dirty solution for this, would be communities having “tags” in their metadata. This wouldn’t prevent spam, or an accumulation of four trillion tags, but you could easily add “only these tags,” or “not these tags,” to any feed. User objects have metadata that is used like this (as the “bot” flag) already. I’m just familiar enough with the code to know it wouldn’t be a slam dunk, but it’s also not a breaking change or re-write!