There are two such accounts, as found in T346465: Data about registered users who share the same format as temporary accounts will.

Some discussion about how to do this can be found on T300265: [IP Masking] Inform username accounts prefixed with "*", created when we expected to rename every account starting with *.

However, since there are only 2, perhaps they could be notified individually.

Timeline: since this is a blocker for T348895: [Epic] Temporary accounts testwiki deployment, which is planned for March, the users will be asked to pick different names before then, minus some time buffer.