Good question! Tire speed ratings initially only went up to the "Z" rating, which (at the time) was just "In excess of 149 MPH". Additional ratings [W= 168MPH, Y=186MPH, and (Y)= in excess of 186MPH] ratings were subsequently added to further define the ratings. There really isn't any such thing as a "Z" or "ZR" speed rating anymore, but you may find it as part of a tire size (e.g. 245ZR-18) However, the rule book (Section 2.F in the 2020 edition) continues to list "VR" (the "V" speed rating is actually 149MPH) and "ZR" tires as being good for "Up to 200MPH". I'm guessing the "R" stood for "radial". I submitted a rule change on this topic a couple of years ago, asking them to adopt the actual, current ratings, but it didn't happen. So, if you go by what the rule book says (Section 2.F in the 2020 edition), VR and ZR are still good to 200MPH, and (since they're not listed) tires with W, Y, and (Y) ratings are unacceptable for anything.
Jeff in Boise