Debian sidでパッケージ更新をすると新しいaptがやってきました．そしてapt-listchangesで気になる記述が表示されました．
$ zcat /usr/share/doc/apt/NEWS.Debian.gz | head apt (2.9.11) unstable; urgency=medium The ftp, rsh, and ssh methods have been removed. They have been unsupported and disabled since 1.8. Please, migrate to http(s) instead, or contribute an sftp method. If you need ad hoc access to a remote repository, you can usually run `python3 -m http.server` on that machine and use SSH port forwarding to run HTTP over SSH.
ftp, rsh, sshメソッドが削除されたようです．