It's an alternative skin/user interface that fediverse instances can offer users. It turns out to have an internal blocklist of instances, communities, users, and keywords that it will silently hide. Some (e.g. child abuse) are understandable. Others (e.g. specific usernames) are less so.

Some people are upset they are on the list; some that they are not on the list. But most are upset that a list exists in the first place.