I've looked through a few threads from many years ago discussing the possibility of designing maps with one-way connections. I'd like to ask if anyone has followed through with this and made a one-way connections mod or something. Do the more experienced players know what happened to this idea? Did it ever come to fruition? I'm not sure what I would use that feature for but I am curious to see if other players have already looked into it. I might like to make a novelty siege map or something that would use one-way connections if the concept has already been proven.

There is a mod called "One Way Connections", which allows game creators to make one-way connections

Does that mod allow players to designate one-way connections themselves or can they only use maps made specifically for that mod?

The game host configures the one way connections before they create the game. This can be done regardless of the map. (But since it works based on territory names, it is a bit clunky.)

I see. How does it affect gameplay? Does it lead to interesting outcomes? Are there any maps specifically designed to take advantage of that mod?

Entirely up to game creator. Affect of outcomes is uncertain because of so much possibilities.

But the mod should really use territory ids, not names, to make sure the correct territory is affected. Territory name is good as a search, but not for precise territories, if territory names aren't unique.