When a player discards their clothing, they are barefoot.

A foot strinking the ground - irrespective of surface type - will always make less sound than a boot striking the ground. This does not currently appear to be the case, however.

In my opinion, the realism of this game is exponentially boosted if men in underwear produce significantly less noise when walking and running, as compared to men in heavy combat boots.

I am only familiar with the Battle Royale mod, on which I have extensive experience in discarding clothes and playing in underwear. I cannot speak for other game modes or mods.