we had an issue related to incorrectly caching 404 responses earlier but that was a few hours before your post. it should already have been resolved when you posted this.
sorry about the late response, got a lot of stuff going on currently and it seemed like you got useful replies here already anyway when i checked before.
we currently have a rule in place that blocks traffic with too high of a threat score. this rule was implemented before i joined, i'll have to check with the team about the original reason for this and if we want to relax this.
at least the error message should be improved if we can do that, i think that's just returning a static message currently.
while this is generally the intention, unfortunately we're not always there yet.
i know that lemmy <-> piefed already works, but private messages between lemmy and mastodon users for example do not currently work, and there is also currently no indicator that those messages never were delivered. for lemmy <-> mastodon, this is tracked in https://github.com/LemmyNet/lemmy/issues/2657 and there is work going on right now to resolve this incompatibility.
we had an issue related to incorrectly caching 404 responses earlier but that was a few hours before your post. it should already have been resolved when you posted this.