[wizzwizz4]

It's possible to contact the instances who retain the copies of the data, and have them delete it. It's not like the Fediverse is some kind of blockchain, where data is replicated forever unless authorised parties delete it. (And deletion requests can still be sent out by whoever controls the domain, after the instance is gone.)

[nagaiaida]

(to be clear, i have been on mastodon/fedi for coming up on a decade now and have no qualms about the way leaving or sunsetting an instance works. part of the magic imo is that old posts from long-dead instances regularly go around again)

how exactly do you propose someone looking to delete all of their old posts from a dead instance might obtain the exhaustive list of instances that have historically received their posts via a long indirect cascade of boosts to send these formal or informal deletion requests? an allowlist-federated instance obviously makes that far more tractable, and otherwise I could potentially see annoying paths to doing it anyway if i had been the admin and had every log ever sitting somewhere to trawl through, but in the case of a normal user whose instance is already gone and who doesn't have an ongoing relationship with the prior admin, this is hardly a realistic way to address that concern.

[wizzwizz4]

Obtaining the exhaustive list of instances is very difficult, but typing your old handle into the search box of the ten largest Mastodon instances will get you many leads. Aiming for an exhaustive list is somewhat security theatre: chances are, many of your Fedi posts are in Common Crawl, which the instance sending DELETE wouldn't affect – and good luck redacting Common Crawl. Fedi is not a secret medium. (This is not to say that the maximalist "if it's technically possible, I have a right to do it!" position is anything other than a huge violation of privacy.)