Probably since America was invented
The following hits the nail right on the head:
Other GA IFR pilots of non-pressurised aircraft that populate the FL100 to FL150 levels will be well familiar with this problem. Where will the cloud tops be? … dunno, there’s much less guidance available for your route than a real forecaster could give you (and, actually, less still now the new Met Form 215 doesn’t even bother with altitudes of 10,000 ft and above). Will I get moderate or severe icing in the cloud if the temperature is below 0 degC? … absolutely always, according to the Met Form 215, despite the fact that, more times than not in stratiform cloud, you won’t. Embedded Cb's? ... TEMPO, oh, that's helpful. And so on.
Actually F215 forecasts icing in
all cloud, not just below 0C
Very helpful (not).
Fortunately, there is GFS. One gets TAFs/METARs and MSLP and SIGWX charts from the usual aviation weather services, and then looks at GFS for the more detailed stuff. But it's changing, with more websites doing graphical presentations from GFS.