Counter question, what is the use case here?

I have only ever needed this feature with NSFW content. In Lemmy I have easily and better way resolved it by having another account with nsfw instance. This creates even better outcome than the multi community feature. All clients easily support two accounts, and you can switch between them in few presses.

If archive.org, or any other web scraper is able to pull personal information from a site, it means that the site is already breaking the GDPR.

GDPR protects personal information, not public texts.

Because instance holds identifying information about EU citizens (email, nickname), it means that the instance owner is the registery holder, and they must comply with GDPR.

I believe email address of the user is not shared between the instances, what makes things quite good. Nicknames are bit more problematical, because they can be considered as personal identifier.

Some GDPR experts maybe should write template registery document that instances can use. And the delete of account should be handled between instances. Posts do not need to be deleted, but nick should be changed to [deleted]

That quote from GDPR talks about specific job role that large company is by-law requires to have, called data protection officer. He/She is responsible that company is GDPR compliant.

This is incorrect, GDPR is any registery, company size or even profit/nonprofit is not relevant. Even it being digital/in paper is not relevant. If EU citizen is identifiable in registery, it must comply with GDPR.

Lemmy clients could have a logic that if same username from multiple instances is in same comment tree, it would show the instance. This way another user cannot impersonate in same conversation

Few exceptions are valet parking and Uber, both of those also rely on tips a lot