Mastodon uses webfinger for this, which scales much better (though I find it annoying webfinger wasn't specified with a url format that'd make it easy to provide static replies like what you suggested), and supports aliases and redirection.
E.g. if you go here:
https://galaxybound.com/.well-known/webfinger?resource=acct:...
You'll get redirected here:
https://m.galaxybound.com/.well-known/webfinger?resource=acc...
Which will return a webfinger profile, which includes this activitypub profile (curl command line because you need the "Accept:" header):
curl -H "Accept: application/activity+json" https://m.galaxybound.com/users/vidar
If you look at the "publicKey" key in that JSON doc, you'll see:
{
"id": "https://m.galaxybound.com/users/vidar#main-key",
"owner": "https://m.galaxybound.com/users/vidar",
"publicKeyPem": "-----BEGIN PUBLIC KEY-----\nMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAvHAMudraHmCA2u8wcI2/\n7iB0Td73AF7tk3LC7AuUxzCMu0Ipf/mNVdh+2nFOQ0pY+7E/wEgGeFg5BlMNC7hK\nebfYtY5ZFm7upVqQ0OXTP2hC+fnDTNcpmnPqmwMRdL54YtiZvQtRzW++ZKquWjER\nInM97NsjW0H0yuVkMETfPX1ilWkzTlWa9m0+H5Tmaz3EbKk3VanJXSLKNLnSq2lK\nbKcr2kD/U5UobzJDaaKjK+tW50iTOMiFEXLAT+4Po375WlgxKchahvtPyioiDm6j\noneGAbKRM/eAiHf7EhP76zyoL/LNG0XP4rDEFr4Ia4vo4HG1zhbGZ+815Tip4lNW\nYwIDAQAB\n-----END PUBLIC KEY-----\n"
}
.. which is used to check the signature of my posts when they federate.
From the profile you also know where to get my posts, and can get them either as JSON or RSS if you want to pull them without actually following me.
If someone "moves show" there'll be an alias in place. E.g:
curl -H "Accept: application/activity+json" https://mastodon.social/users/vidarh | jq .movedTo
gives:
"https://m.galaxybound.com/users/vidar"
Now, something like a DHT to replace webfinger so a move can be done without cooperation from the original instance might be interesting, but Mastodon works pretty well here already.
