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Can git.io links point to a server that's not controlled by GitHub though?

Not directly. According to github, only `github.com` and `*.github.io` links were enabled.

But one could set up an html on github pages that redirects to the 'real' URL.

Not only 404 but also 5xx errors.

As an example, I run an online store, Google checks the landing page urls for ads once a day. If you happen to have a temporary outage (surprisingly common on Heroku) at that exact moment your ads will be deactivated and not run for up to 24hrs!

I’m somewhat suspicious it also dumps your ad back into a learning phase.

And learning the target url allows you to look it up on archive.org, even if the target is permanently unavailable.