There are a few things to consider with that, seems the process should be able to be customized by site admin depending on all the specific circumstances.
If someone is requesting full anonymization there are the gdpr policies, if the username is their actual full legal identifiable name then seems that would probably need to be removed if quoted by other user members especially on public site.
However even just that wouldn’t necessarily be enough to complete the anonymization process depending on what is being posted in text along with the username quote, ideally if that can be reviewed by site mod/admin to make sure there isn’t personally identifiable information in that also could be important.