OpenSSH is indeed widely used. But the only game in town they're not. There are commercial SSH implementations that aren't based on OpenSSH, reasonably widely used too. For decades I've used a Windows product called SecureCRT based on their own ssh and sshd stacks at Van Dyke. I'm sure there are others too. (I hated putty's UI.)

There's an obvious infosec observation here. Different stacks, different hacks. Maybe individual infosec people should adopt a randomly chosen stack from among the less popular. Then the dreaded zero-day that hits the most popular stack won't cut off their access entirely.