ssh-connection’ protocol
In addition to SSH itself, PuTTY also supports a second protocol that is derived from SSH. It's listed in the PuTTY GUI under the name ‘Bare ssh-connection’.
This protocol consists of just the innermost of SSH-2's three layers: it leaves out the cryptography layer providing network security, and it leaves out the authentication layer where you provide a username and prove you're allowed to log in as that user.
It is therefore completely unsuited to any network connection. Don't try to use it over a network!
The purpose of this protocol is for various specialist circumstances in which the ‘connection’ is not over a real network, but is a pipe or IPC channel between different processes running on the same computer. In these contexts, the operating system will already have guaranteed that each of the two communicating processes is owned by the expected user (so that no authentication is necessary), and that the communications channel cannot be tapped by a hostile user on the same machine (so that no cryptography is necessary either). Examples of possible uses involve communicating with a strongly separated context such as the inside of a container, or a VM, or a different network namespace.
Explicit support for this protocol is new in PuTTY 0.75. As of 2021-04, the only known server for the bare ssh-connection protocol is the Unix program ‘psusan’ that is also part of the PuTTY tool suite.
(However, this protocol is also the same one used between instances of PuTTY to implement connection sharing: see section 4.17.5. In fact, in the Unix version of PuTTY, when a sharing upstream records ‘Sharing this connection at [pathname]’ in the Event Log, it's possible to connect another instance of PuTTY directly to that Unix socket, by entering its pathname in the host name box and selecting ‘Bare ssh-connection’ as the protocol!)
Many of the options under the SSH panel also affect this protocol, although options to do with cryptography and authentication do not, for obvious reasons.
I repeat, DON'T TRY TO USE THIS PROTOCOL FOR NETWORK CONNECTIONS! That's not what it's for, and it's not at all safe to do it.