While they definitely do this for handles I’m pretty confident this is also done for DIDs (Decentralized identifiers) and it doesn’t provide a solution if you lose your domain. I think Bluesky (Appview) specifically gets around this by also tying your DID:web to your DID:plc, in case of domain loss. So I think it exists on the protocol but they don’t automatically utilize the decentralization for end-user experience(domain loss) but other appviews can. But I could be wrong.
Mastodon is a far worse experience than Twitter and Bluesky, it’s not a 1:1 transition.