I am not with Laminar Research; just a flight simmer who does not support steam.
X-Plane is not a game. It is a flight simulation package.
Steam is a 3rd party hybrid that is taking a commercial advantage of all of the development work carried out by Laminar Research. It is not the same product as the official original version created by Laminar. As I said it is a hybrid. Any updates by steam are slow whereas from Laminar their updates are immediate including Beta etc versions.
LR does not sell or support steam as the original product does not mix and match with Steam. There is no connection between both parties.
As Thomson (from Laminar) has indicated it can be purchased directly from them. Here is a more detailed link to purchase XP10 found at https://www.x-plane.com/product/x-plane-10-digital-download/
When you purchase the digital version X-Plane 10 it will be the final version. There will be no further updates as all work is concentrated on X-Plane 11.
Support and purchase the official and original product which has a direct benefit of encouraging further development by Laminar Research of X-Plane.
Why not go for XP11. It is easier to use and has a lot more benefits than XP10
Good luck and welcome to X-Plane with your purchase from Laminar Research.