So noticed a few hours ago the feeds had changed.
However, each instance seems to behave differently from one another. Would have posted on Github if I could reproduce the errors consistently.
Here on TBB, any feeds give an unsupported format error.
On Kbin.Earth, the feeds give a 403 error instead.
On GehirnEimer, which I don't have an account on but can be browsed without an account, feeds work fine.
Tested with: