Which Forza and which LTSC version?

I have not had issues with Horizon 5 or the new Forza on my steam deck under Windows 10 LTSC 2021. I believe I played both at or near launch.

That is the first I have heard of a game not working on LTSC.

Edit:

My timeline is off, I did not try Horizon 5 on my steam deck until about a year after it came out since Horizon 5 predated the steam deck.

However worth pointing out that it is one example of a game not working on LTSC vs many examples on proton so I would argue it still Wins out on compatibility.