Short plates and uncoated gray plates are indicators of the general trend towards cost cutting. In the span of a decade or 2, focus shifted hard from making the "best" quality tubes, to simply making them cheaper.
Some of these changes you can see via less materials and simpler construction: e.g. from welded to stapled plates in EL34; no more 5751 triple micas or retaining "clips" securing the plates to micas, no more massive "mirror" flashing on Sylvanias, loss of copper grid posts and heat sinking pieces. Some you probably can’t see (chemistry, metallurgy).
If you compare Mullard short plate 12AX7 to long plates from just a few years before, the long plates should sound a little more open, holographic, and articulate. The short plates just sound warm & thick, which is sometimes all you need from Mullard I guess. If you go a bit further back you’ll find long plates with the D / square getters which are even more revered.
Short plates can sound great but I think often if you can compare to the comparable long plates from just a few years earlier, you’ll prefer the latter. Bigger plates can mean more microphony too, though. The way these things are suspended in the glass envelope makes them a vibration antenna, and the mica spacers have to retain a tight grip over all these years & heat/cool cycles.
