Hello,

I decided to enable SSH server on the RB750Gr3 router, using 6.42rc39 build, but the /system ssh and /ip ssh commands are not accepted. Before buying this router the spec sheet of this model stated SSH on most websites that sold it.
Does it support SSH server/client at all?

You need to also enable the server
See /ip services

https://wiki.mikrotik.com/wiki/Manual:IP/Services

via - as far as client, I don't know, I have not used
Mikrotik's server based client.. port forwarding, etc. But the basic functions are there.
You can ssh into it. It behaves properly.

It does SSH fine. Server and client.

Found this..........
More Secure SSH access
RouterOS utilises stronger crypto for SSH, most newer programs use it, to turn on SSH strong crypto:

/ip ssh set strong-crypto=yes

indeed, i had security package disabled that's why ssh was missing.
thanks guys !