Here is a pit with a normal floor set at -64 and a nice glowing flat on the floor:
Here is a similar pit but the floor is actually -128 this time and a type 209 fake floor is being used to put the green liquid at -64. It still glows. i.e. the pit looks identical to pit 1 until you jump into it and go below the floor.
And finally, here is basically the same construct as number 2, but made using a 3D floor. No glow.
So, the generally easier to set up and more reliable 3D floor method doesn't look as good as the fake floor method (and it also means no glowing translucent 3D floor liquids).
I know that the glow can be faked using the floor glows sector property in UDMF but, from testing, this only works if you put the glow on the containing sector. So, in an example like the above, I would have to make the glow something like 128 tall to account for the area below the 3D floor and to put the glow above the liquid surface too.
I mean, it looks nice enough:
Ooooh, translucent too
But I have to fiddle around with the values far more than I would if the the GLDEFs preset glow was used (e.g. here I had to set a height and pick a glow colour quite different to that of the GLDEFS one I'd used to make it look similar) so, if it's possible, could 3D floors that use the GLDEFs glows be something to add to the engine?