Forums like these still exist in Germany. GDPR compliant. And from what i gather Germany is comparatively restrictive with data protection laws.
You need to consider that the GDPR does *not* prohibit any and all data collection.
If you have a legitimate need for an information, like the email address for registration as a way to verify and recover accounts, it is allowed to store it. What you cannot do is use that information for anything else that takes your fancy, like selling them to some information broker or ad company.
For deleted users the posts stay, and in all cases i remember the username also was kept associated to the posts, but the account information linked to the username is gone. Just prohibit using the email as actual username, and you are safe in that regard.
It can get a bit ugly though if someone used their real name as handle. I have seen it anonymized as "user<randomnumber>", but quotes are often not retroactively changed. I do not know if that might even be a GDRP issue though, since i don't think it is a reasonable expectation to do a "search & replace" through the whole forum.
Considering that at least one "User purge" that also removed most posts of that user happened here on the EEVBlog, i do not see what the effects really would be though. There is precedence, after all. Also I would not expect that such full purges are something that would often get requested here, since hopefully most users here are rather reasonable.