I somehow didn't think about that, even if I used that feature just a few hours ago! Now I'm curious about how GitHub handles the ssh infra at that scale...
GitHub, as I've read[1], uses a different implementation of SSH which is tailored for their use case.
The benefits is that it is probably much lighter weight than OpenSSH (which supports a lot of different things just because it is so general[2]) and can more easily integrate with their services, while also providing the benefit of not having to spin up a shell and deal with the potential security risks that contains.
And even if somehow a major flaw is found in OpenSSH, GitHub (at least their public servers) wouldn't be affected in this case since there's no shell to escape to.
[1]: I read it on HN somewhere that I don't remember now, however you can kinda confirm this yourself if you open up a raw TCP connection to github.com, where the connection string says
SSH-2.0-babeld-9102804c
According to an HN user[2], they were using libssh in 2015.
[3]: This isn't a value judgement on OpenSSH, I think it is downright amazing. However, GitHub has a much more narrow and specific use case, especially for an intentionally public SSH server.
