Back in the 90s it was hard to find the clear answer but I did find it in writing somewhere, maybe in their book/catalog "Lens Work II" from around 1997. Their official definition was, from memory, that it had GROUND aspherical (not glass-molded aspherical, abbreviated as "GMO"), Super UD, and/or Fluorite elements. The definition they gave surprisingly did NOT include "pro build quality" or "luxury" as part of the meaning. However that's not to say you're wrong: I'm sure their definition changes every couple years, if they even have a definition, and ultimately, the most accurate definition of what "L" lens is, is the list of lenses that say "L" on them.
I won't go looking through the internet to find data points, but I THINK Super UD has moved down-market to the point maybe many "consumer" lenses have it. Meanwhile I THINK GMO might have moved up-market as they've gotten better at forming it. It's possible that the best differentiator now is whether or not it has weather sealing: I wonder if all L lenses do and all non-L do not? And yet that may change too: why NOT put that little rubber gasket on a 50/1.8 say?