Most likely they just don't want to continue having to support it at all going forwards.
Tech debt is a big issue with codebases as time goes on. Steam isn't necessary, and usage likely wasn't very high, so the time to maintain updating it was likely determined to not be worth it. So you end support where it's at with the currently deployed user base and let it die naturally.
That doesn't have any bearing on whether Tesla wants to officially support it on more vehicles.
If something breaks, people will blame Tesla first, even if they changed absolutely nothing. The average person has very little technical knowledge and essentially thinks of all of their electronics as a magic box that just works, until it doesn't. They don't care about the how, the why, or the who, they just care about whether it works or not. And if it doesn't, the first person they blame is whoever makes the box, even if they have nothing to do with the software the person was using.