A feature that fairly consistently emerged (again?) after a few years was the aforementioned elastic "stirrup" at the bottom of the pant legs, which wrapped around the boot at the heel and kept the pant legs looking nice and sharp (preventing the pant legs from pulling upward or twisting much).
This was hardly a new practice, as evidenced in R.I. Davis' wonderful book, Men's Garments 1830-1900:
Although used previously on TNG, the VOY jumpsuits didn't seem to have this feature the first few years.
|DS9, 3x8 "Meridian"|
As best we could tell, they did start to creep in during the 1994-1995 production year - that is, Deep Space Nine's third season, Voyager's launch, and Generations (the three examples above notwithstanding):
|VOY, 1x13 "Cathexis"|
|DS9, 5x4 "Nor the Battle to the Strong"|
They seem to have been inconsistently employed, though, as you can see from the following screencaps (all from Voyager's fifth season):
|VOY, 5x19 "The Fight"|
'course, removing them for "biobed scenes" would hardly have been an unsurprising approach (again, B'Elanna's example in "Nothing Human" above notwithstanding).
|VOY, 1x7 "Eye of the Needle"|
Or, perhaps it was only done for characters/actors who had boots with taller heels?