Aesthetically, are jumpsuits supposed to be a reoccurring feature of Demacian dress or an accident of convenient design? Is it something that will be moved away from in future updates to their designs?
A lot of champions from there seem to wear jumpsuits, almost catsuits, under their armour. At least that's true for a couple of older designs.
Edit: It seems someone already put up a similar thread a while ago. Woops!
Maybe they're just comfy. You need something to pad up against all that heavy armour.
TLDR: Yes, but Riot is open to replacements.
Long Version: Yes! Whereas this may just be my opinion, the suits offer a nice balance between protection and mobility, which is why the armor they do wear are typically protecting vital areas, and areas that aren't movable joints. It's kind of like under armor for athletes, or the suits spartans from Halo wear under their armor. Currently, jumpsuits are the norm, but this is in no ways set in stone! Riot has openly looked to the community to see options they believe may be better. Personally though, I'm a fan of the jumpsuits.
Don't forget 
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It's got to be the reason his shoulder pads are so freaking huge.
It shows the Demacian physique, telling its enemies that their bodies are there better than the enemies.
I always thought it was soft leather or chain mail. But looking at it again most of that stuff looks like latex. :/
BUT WAIT.... VALOR DOESN'T WEAR A JUMPSUIT.... ILLUMINATI CONFIRMED?
Well, in Fiora's case, I think that's Riot's take on a Demacian fencing outfit of sorts.
For Quinn, scouts usually wear light/leather armor, so she's very lightly armored as a matter of course.
Lux...um... ...ok, there's really no good reason I can think of. As a mage, wearing even light torso armor like she is really doesn't make sense for her to have that blue catsuit underneath. Rito, can we have a new splash for her, plzkthanx?
Vayne...meh, leather-clad vigilante is exactly what they were going for with her. Mission accomplished.
