Why would they purge SSH keys when they don't purge anything else? Why not just purge the whole account after a year of inactivity, if they care so much about space?
It's clearly not about space. Old SSH keys are a security hazard. Even moreso keys you aren't using anymore and therefore may not be particularly careful with.
Heck, even in this very scenario, if I haven't used an SSH key with GitHub in many years, and then GitHub receives an artifact signed with that key saying "I lost my 2FA token and backup codes, please reset account auth so I can log back in", I very much do not want GitHub to trust that artifact. If I haven't used the key in years, that probably means I don't have it anymore and either never got around to removing it from GitHub or forgot it was there.
Heck, even in this very scenario, if I haven't used an SSH key with GitHub in many years, and then GitHub receives an artifact signed with that key saying "I lost my 2FA token and backup codes, please reset account auth so I can log back in", I very much do not want GitHub to trust that artifact. If I haven't used the key in years, that probably means I don't have it anymore and either never got around to removing it from GitHub or forgot it was there.