Does dropped loot and ammo ever decay?

Solution 1:

While I have never witnessed drops disappearing, I have had a suspicion that they do, especially considering scenarios where you comb the entire map right before the end, and still end up having received considerably fewer drops than someone else in the squad. This would not happen if drops never decayed.

Also anecdotally, there seems to be a feeling in the community that they do indeed despawn.

I was not able to nail down an official source, but the thinking appears to be that there is a cap on the number of resources on the ground at one time. It would seem that after 50 items are on the ground, the 51st item will cause the 1st to despawn¹.

From a programming perspective, this makes sense, as infinite new entities have a tendency to bog things down. This also explains why this phenomenon seems to be so hard to notice, as by the time you've let 50 other drops pile up you've probably completely forgotten about the first one. This also means that if the mission only drops <50 items, they do stick around forever.

¹Of note, different categories of loot seem to have different "counters." Meaning health orbs, for example, won't count towards your maximum amount of displayed resources and so on.

²Also of note are these patch notes, which say

Mods and Blueprints will never expire as drops.

which implies quite strongly that other types of drops do expire.

Solution 2:

Having played Warframe for 484 hours, my experience is that all drops will persist for the duration of the mission unless they dropped off of the map.

Barring any bugs, this seems to include when host migrations are done. Then again, if I understand Warframe's technology correctly, the host copies the entire map state to all other players so that anyone can switch to a host at any time.

Note: Plains of Eidolon tends to be buggier about this with host migrations than standard areas are, likely due to the area's size.