Public key authentication is actually a Must Implement for SSHv2. Since SSHv1 is long obsolete, any gear that doesn't have pubkey doesn't actually have a de jure SSH implementation.

"All implementations MUST support this method"

That doesn't mean it's always easy to install and manage keys. For example, the author of the passh tool recommended by this post somehow managed to come away with the impression that OpenWRT's ssh server only supports password authentication.

Another example: Ubiquiti gateway consoles like the UDM-Pro. You can install an SSH key but these are erased on reboot. So after every reboot I have a script that uses the SSH user password to re-install an SSH key but this can’t be relied upon and I haven’t found a way to make an SSH key persist.

Here you go:

https://github.com/fire1ce/UDM-Persistent-SSH-Keys

Dell PowerConnect 5500 series has a very picular SSH implementation, which could be described as 'allow all SSH proxy for telnet'

And if you don't, anyway? Do you not get to use the SSH(TM) logo on your product? You're reading MUST a bit too literally.

Exactly as I wrote, it means what you've got isn't a de jure SSH implementation.

Do with that whatever you will.

That doesn't mean that a device that doesn't offer pub key storage is not accessible over SSH.

Yeah this is why fido2 doesn't work either. Most embedded ssh implementations don't support it.