Old-school forums have single points of contact. They're no more private than ActivityPub, but a takedown to the admin is a takedown of all instances. Obviously public data can be cached or archived, so as always you have to send takedowns to every archival service, search engine, and any CDNs too.

The GDPR "applies" whenever an EU resident's data is stored. The enforcement requires some presence in the EU by the entity storing the data. For multinational companies that means if they have any banking services there (e.g. taking payments from EU customers) they have a presence. For individual fediverse admins, that's not necessarily a concern. At worst their instance's domain would get blacklisted to EU users.