mastodon.zunda.ninja is one of the many independent Mastodon servers you can use to participate in the fediverse.
Zundon is a single user instance as home of @zundan as well as a test bed for changes of the code.

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呼び出し側は

after_key_change! if key_changed? && !@options[:signed_with_known_key]
clear_tombstones! if key_changed?

となっていて墓は暴かれるけどsigned_with_known_keyオプションが有効な場合にはフォローしなおさない?

joinmastodonのドキュメントをざっと読むと基本的には現在の公開鍵を投稿元のサーバからもらってきて署名の検証に利用し、投稿元のアカウントが既に存在しないDeleteアクティビティを受け取った時だけローカルにキャッシュしている公開鍵を検証に利用するようにも見える。ふむふむ。
https://docs.joinmastodon.org/spec/security/

docs.joinmastodon.orgSecurity - Mastodon documentationPublic key cryptography and supported signature schemes over HTTP and JSON-LD.

@Sujiyan Yep. My experience tells me that I could re-follow an account with the same @username@FQDN recovered on a totally independent installation of Mastodon.

@zundan So you mean a cryptographic UUID is needed?

@Sujiyan No. I guess this just means Mastodon is designed to identify accounts with just their URI (username@FQDN). Key pairs are only for verifying the authors when posts are transferred between servers.

To verify a Mastodon account from something else, one can add a hyperlink from there back to the profile page on Mastodon with rel="me" attribute.