Comment by j1elo

Just for those curious about it:

  $ ssh whoami.filippo.io

      +---------------------------------------------------------------------+
      |                                                                     |
      |             _o/ Hello!                                              |
      |                                                                     |
      |                                                                     |
      |  Did you know that ssh sends all your public keys to any server     |
      |  it tries to authenticate to? You can see yours echoed below.       |
      |                                                                     |
      |  We tried to use them to lookup your GitHub account,                |
      |  but got no match :(                                                |
      |                                                                     |
      |  -- Filippo (https://filippo.io)                                    |
      |                                                                     |
      |                                                                     |
      |  P.S. The source of this server is at                               |
      |  https://github.com/FiloSottile/whoami.filippo.io                   |
      |                                                                     |
      +---------------------------------------------------------------------+

Of course, this happens because the day I learned about the default behavior of SSH (to send all your keys in hope that one works), I went ahead to disable it to stop remote servers from being able to inspect all my keys. I feel this can be abused in a similar way that sites abuse browser information to fingerprint users. So I put this at the bottom of ~/.ssh/config:

  Host *
      IdentitiesOnly yes

And then I explicitly indicate what key to use for each server, either with the "-i" argument, or adding entries above the previous lines:

  Host example.com
      IdentityFile ~/.ssh/example.com.pem

Other commenter mentioned that something similar can be achieved with "PubkeyAuthentication no", but I've been using "IdentitiesOnly yes" for years without issue.