You could give us an SSH key when you ordered a VPS, and after that there was
nowhere to add a second one — a new laptop, a colleague, a CI runner. Every VPS
now has an SSH keys tab where you can add and remove keys yourself.
Keys are written onto the server by cloud-init, which only runs when the server
is built or rebuilt. We say that on the page rather than letting you find out,
and we give you a copy-paste command that authorises the same keys on a server
that is already running.
The bigger fix is underneath. Reinstalling a server used to hand it back with
whatever login it was originally built with — for anything built before we
switched to a sudo user, that meant root with a password. A reinstall now
writes your current keys and a fresh sudo login before the rebuild starts, so
the server comes back the way your portal says it is. If we cannot do that, we
refuse the reinstall and tell you, instead of quietly rebuilding you into
something else.
Windows servers are left alone by all of this — they do not run cloud-init, and
we would rather show you nothing than a login that was never created.