>OpenSSH (and it's ancestor ssh-1.x) have a 17 year history of using ~/.ssh. This location is baked into innumerable users' brains, millions of happily working configurations and countless tools.
XDG can be considered as a case of arbitrary change for weak justification. SSH is definitely a case where people need to know exactly where the configuration is. There is no point in including it in the not very fun "find the config files" game engendered by XDG.
However, an issue I have is that while you can configure that path system-wide, there is no way to control it per-user, or with a finer-gained approach. You can probably use mount namespaces to shadow a ~/.ssh with another config, but that seems overkill.
I admit I have no practical use cases for this right now (though I probably did in the past) besides "uncluttering $HOME". As for hunting the config files, those would reside in ~/.config/ssh by default. I personally find it more irritating when a program picks a random folder instead of conforming to the spec (and if I set the environment variable to somewhere else, I then know where it is). Go hunt trough ~/{.program,.program/conf,Program/conf,Documents/Program/conf,Documents/My\ Games/conf}, or countless variants I experienced. Thought it is admittedly a much smaller problem with better-established programs such as openssh.