We're restricting most slurs with the username filters and the team is currently cracking down on methods of getting around those filters. As far as some that have been allowed thus far, they are ones most frequently reclaimed by the community, and even still we are restricting usage to be only referring to yourself. Referring to anyone else using a slur is still ban-worthy, like allie said above. I hope this is acceptable to you and we'd love to hear future thoughts on the matter and we'll take the community's thoughts into account as we continue to discuss what should be allowed. The cases you've seen already we've almost certainly seen too and have ongoing discussions about whether or not to change their usernames.
Also please feel totally comfortable using the report feature on any posts or users that you are worried about, that will bring it to our attention the fastest and won't do any harm if they are found to be within what's acceptable!