Wait, how often do you connect to a ssh remote that isn't controlled by you or say, your workplace? Genuinely asking, I have not seen a use case for something like that in recent years so I'm curious!
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.
[2]: https://news.ycombinator.com/item?id=39978089
[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.
This might be similar to the workplace scenario that you have in mind, but some students are more bold in trying dodgy things with their class accounts, because they know they probably won't get in big trouble at an university.
Nobody is ever getting in, because they require ed25519 keys, but it is pounded nonstop all day long with brute force attempts. It wastes log space and IDS resources.
This is a case that could benefit from something like the new OpenSSH feature (which seems less hinky than fail2ban).
Another common case would be university students, so long as it's not applied to campus and local ISP IPs.