It probably can’t be removed completely, since some add-ons require users to enter their SimBrief details into the FMS. Since the FMS has a limited keyboard, users with special characters in their usernames wouldn’t be able to enter them properly.
I think the Pilot ID can be kept as a workaround for these situations. But I agree it should be discouraged in general, I’ve updated the Fetching a user's latest OFP data post accordingly.
Cheers,