I've come across claims from modern nudists, that an apparently common scenario of farming naked in the ancient world, was that, as described in the title, they were very poor and it was actually an accepted reality that it was important not to wear out one's clothes unnecessarily when working hard in the field. I have read that in various ancient settings, one may have had only one pair of regular clothes, and possibly a second one for special occasions only.
There might also be other scenarios (outside of poverty and necessity), that prompted naked farming and I think it would be an interesting subject to find clarification on.
What evidence of nude farming in the ancient world (in civilizations such as the Egyptian, Greek, Roman, Hebrew, Assyrian-Babylonian) do we really have to support this claim, and what, if anything do we know the reasons for it?
Also, if any knowledge exists on the topic, is it clear whether there was gender separation in such settings, for 'moral' or 'modesty' reasons?
this claim does not make much sense.
First of: the paintings and sculptures depicting ancient peoples are symbolic and do not describe actual reality of their work. The Nudity here is exaggerated for effect.
Two: the ancient civilisation you described inhabited a very hot and sunny part of the world. While it may seem logical that many of them would work naked, the reality is that in some of those times and regions (like ancient Italia or eastern Greece), doing so would result in an extreme sunburn, overheating and possibly a sunstroke [edited, thanks MyNewAccount27].
However, are at least partially right, in that pre-modern farmers would very often work barefoot, and sometimes pants-less only in a tunic/shirt. For most of human history, nudity was not as controversial and a "taboo" as it became in the XIX century and partially remains to this day. For most of the Antiquity and European middle ages people would bathe communally (and often both genders together) share beds, women would brestfeed publically, and both genders would urinate and defecate nearly publically without much shame. So, nudity, and exposition of sexual organs was not a taboo as it is today.