Resolved by adding the following to the server-side sshd_config file:
KexAlgorithms diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
Note that the full list, including contemporary defaults, would be:
KexAlgorithms curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
The UI context menu commands (right click in current folder window, Source Control sub-menu) behave differently from the system level git commands; MATLAB ships with a different version of ssh.