I believe that critical updates on SR+ Model 3's are only done when the vehicle is connected via wifi, at home while charging for example, but I could be wrong. But I certainly hope that all models have LTE and traffic data!
You have it flipped. Their intent is that
critical updates WILL get pushed over LTE to everyone. Non-critical updates would be delivered by WiFi only (for non-premium connectivity).
HOWEVER...there's what Tesla says and then there's what Tesla does. Those are frequently different.
PER THE WEBSITE. Yes, non-premium connectivity is
supposed to mean, no web browser, no netflix/hulu/youtube, no streaming music, etc. However, as of the V10 release, all of this was turned on for the SR/SR+ cars as well. Tesla has
said this is a temporary move (presumably to give them time to figure out how to charge for it), but that remains to be seen. Tesla also said that LR cars ordered after July 1, 2018 would only have one year of Premium Connectivity included, but to date, no one has been cut off.
And yes, as you mentioned, SR/SR+ (ie., non-premium connectivity), is supposed to mean that if you don't have WiFi available in your garage, then you will get less frequent updates ... however, this too seems to be "the same" for everyone. That is, if you are LTE-only, then you don't get many updates regardless SR/LR.
Also on that same site where they say that regardless, the traffic data is used to route you, the difference is in the SR/SR+ you don't get the freeways color coded on the map with traffic.