Dont they have a contract with RedHat to ensure compatibility?

Yes, in my experience, the only thing that doesn't work is the fingerprint reader:

https://forums.lenovo.com/t5/Linux-Discussion/Validity-Finge...

Indeed. Everything works on my X1 Carbon running Arch (via Antergos), except the fingerprint reader.

Thinkpads are great if your standards for screen quality are incredibly low. I can't speak for the ones released in the last couple years but seemingly all of the older ones have garbage tier screens. My T430 has a screen that is at best about as good as the screen that came with my 2009 Asus netbook.

My T460s definitely has a garbage tier screen. And garbage tier trackpad. The trackpoint feels like garbage too. And I had to replace the keyboard once because it was garbage and broke.

You can buy Thinkpads also preloaded with free DOS. (Usually a lot cheaper)

How? Hasn't been an option on the last 2 thinkpads I bought. Just went and tried to customize a T470 and nope, still have to get Windows.

If you're a student, you can sometimes still get it.

You probably can as a business customer? Not sure.

You used to be able to, I don't think you can any longer.