I don't think there is an answer for this. GWRBI (which used to be on the back of Topps baseball cards) may not be tracked officially by MLB but it is still something you can look up.
The fact is once data is out there it can always support a stat. You can't take away a stat unless you take away underlying data. Which is virtually impossible with all the new advanced metrics sites.
Now I could give you an example. Let's say they look at a players caloric input for a day to determine if what he eats affects performance, and then they stop asking players for this info. It would have to be something like that. Player movement, hard stats, all the stuff we have become accustomed to isn't going away. Just because something isn't official doesn't mean no one looks at it.