Anonymizing a user in Discourse retains their username in quoted replies. Patrick_David encountered this issue and questioned whether it was expected behavior. codinghorror confirmed that this has always been the case, citing the expense of replacing quoted usernames. However, Mittineague noted that rebaking old posts can update the anonymized name, but prior names may still be visible in @mentions and notifications. In a related update, gerhard mentioned that with the release of 2.0.0.beta10, anonymizing users now updates their mentions, quotes, and avatars.
Not sure if this is going to suck or not (I feel like it probably will).
I recently anonymized a user at their request. Everything appeared to work as intended with the exception of the retention of the username in all replies where they were quoted. 
Is this expected behavior? It seems like it’ll require some work to really scrub the username association with quotes, but it also feels like it should be done for consistency?
It has always worked this way, the replace would be very expensive.
That’s what I figured. 
If you are interested in globally replacing to anonymize the quotes in other people’s posts for that user, perhaps @techAPJ can write up how to do that at some point.
I did some testing on this a while ago, though I never got as far as to dig into the code involved.
Rebaking old posts will result in the anonymized name being displayed.
But quotes are not the only place where prior names will display. They also remain in @ mentions and Notifications.
@gerhard I just saw your post over on: https://meta.discourse.org/t/ability-to-merge-users/9220/102?u=mattbr
With the release of 2.0.0.beta10 does anonymizing users also update their mentions, quotes and avatars as well?
Yes, it does. See also Anonymizing Users in Discourse