As annoying as that might be, exporting followers seems like a use-case that Twitter might not want to freely support?
His actions say a lot more than all of these re-tweet friendly words he’s been spouting. I’m glad we won’t have to trust people like him or the Twitter Developer team, or need their permission, to succeed in the future.
Who "owns" that relationship? The knowledge of that relationship is necessary for Twitter to run its business, because it has to submit a user's posts to their followers' feeds.
But if I leave Twitter, under GDPR I can ask/make them delete that relationship ("right to be forgotten") but I doubt I can demand that I'm allowed to take a follower's PII, like their Twitter handle, with me to another service.