Re Mastodon: Tbh I’m not sure. I’m also a reddit refugee and have been exploring different fediverse services. I think how mastodon implements it is as a sort of “authenticated redirect”. Not to sure on how ActivityPub supports one account controlling another account on ActivityPub. Maybe could be implemented as a sort of keyring, where the most recent account on the newest instance has tokens for each of the past accounts on the old instances so that their posts can be transparently modified by the most recent account?

Or maybe this is a non-issue and people can just make new accounts on different instances.

is that somewhat new?

it's somewhat… janky.

you can 'migrate' an account, to use the masto term that will make it easier to search. this:

• makes it so your old account can't post
• puts a 'pointer' on that account so that you get its mentions (i think)
• puts a note on that profile that you're really you-at-new-place now
• causes all accounts following you to auto-follow the new place

it does not:

• remove you-at-old-place from other people's follower·ing lists; old you eventually shows as dormant, but you're still in their lists until and unless they clean house
• take any posts with you; you-at-new-place starts with an empty profile
• copy over any profile information
• copy over any post filters

i'm not clear on how long your old posts linger at old-place, and you might have to export/import your following list.

it's possible, i've seen lots of people do it, but it gets more unappealing the longer you've been actively using the account. unless you're like me and have posts set to self-destruct within days. and you can imagine the difficulty of actually moving the posts - if i were an avid shitposter and i moved house to noshit·social, then all my garbage would be dumped in the yard in violation of policy.