alt Hacker NewsThis is half true. If mastodon.social goes down every single one of the accounts made on that instance go down as well. In truly decentralized protocols you own your identity and can take it elsewhere, for instance, in Nostr and SSB, a relay/pub going down is no big deal since you can connect to other servers and maintain communications.
historic posts from the known network and (sometimes media, instance setting) are cached on your own instance in ActivityPub. interactions travel across the known network graph. if an instance vanished forever, overnight, there is at least an imprint of it across the network, albeit instance specific. that may be by design, there are jurisdictions that have people complying with laws and things. not sure how the ecosystems you mention deal with that in particular