Finished some more temperature testing. I'll summarize, and my full notes are in the spoiler.
There are a few features that work or don't work, based on temperature. They are: pathing, wild animal wandering, food stockpiles, meeting places, pastures, and insects.
If all you're looking for is for most things to work as expected, don't go beyond a -90 and/or +100 worldgen temperature setting. If you stay within -90 to +100, most things will work, it's not too risky.
However, there are some pretty cool cutoffs/thresholds you can use to limit or create interesting scenarios for your embarks.
When I make reference to insects/vermin, I mean ants, termites, bees, acorn flies, and similar.
This testing was done with at 299 elevation, with a PSV pocket world that had explicit painted temperatures. The tested biomes were scorching deserts and freezing tundra, respectively. Rainfall was zero in all test worlds. All test embarks were Evil.
The first number in the test results is the worldgen temp, the second value, in 'U, is the measured embark surface temperature (with DFHack 'probe').
If you want to have insects only, but no wild animals showing up, use -103 and +113 for worldgen temps.
If you want to have the most extreme possible temperatures that aren't
instantly lethal (but still pretty lethal!), yet still permit pathing, you can use -1000 to +267 for worldgen temps.
If you want to ensure you have wild animals always showing up, you can use -91 to +104 for worldgen temps.
Of course, if you don't explicitly paint or otherwise specify weights for those temperatures, there will be an even distribution of temperatures by default.
COLD
-75 (9943'U) permits wild animal wandering.
-85 (9935'U) permits wild animal wandering.
-90 (9932'U) permits wild animal wandering.
-91 (9931'U) permits wild animal wandering.
-92 (9930'U) sometimes permits wild animal wandering. Surface stockpiles and meeting places are used/respected. ***** coldest wild animal temp
-93 (9929'U) does not permit wild animal wandering, but permits acorn flies. stockpiles and meeting places don't work on the surface, from this temp and colder.
-95 (9928'U) does not permit wild animal wandering, but permits acorn flies.
-100 (9924'U) does not permit wild animal wandering, but permits acorn flies.
-103 (9922'U) does not permit wild animal wandering, but permits acorn flies. ***** coldest insect temp
-104 (9921'U) no insects.
-106 (9919'U) no insects.
-113 (9914'U) no insects. pathing still works.
...
-500 Animals die on the surface within a week. Extremely fast frostbite / cold damage to all non-metal, non-living things. The rate and magnitude of this damage increases as you go colder from here.
...
-1000 (9250'U) pathing still works ***** coldest pathing temp
HOT
268 (10200'U+) -> 1000 no pathing
267 (10199'U) permits pathing by dwarves and pastures, but no wild animal wandering. alcohol bursts/explosions?/evaporation.. blood explosions in animals dying.. kills everyone on the surface in about 2 days. ***** hottest pathing temp
200 (10148'U) no wild animal wandering. all dead in 5 days. lethal.
175 (10129'U) no wild animal wandering. death in 8 days. reasonably lethal.
162 (10120'U) no wild animal wandering. cat died in 7 days. dwarf died in 11 days.
156 (10115'U) no wild animal wandering. cat died in 17 days.
153 (10113'U) no wild animal wandering. animal fat melting in 26 days. animal deaths within 1 month.
150 (10111'U) no wild animal wandering. no deaths within 1+ seasons.
148 (10109'U) no wild animal wandering. no deaths within 1+ seasons.
137 (10101'U) no wild animal wandering. no deaths within 1+ seasons.
125 (10092'U) no wild animal wandering. no deaths within 1+ seasons.
118 (10087'U) no wild animal wandering. no deaths within 1+ seasons. no insects.
115 (10084'U) no wild animal wandering. no deaths within 1+ seasons. no insects. (also 114 worldgen created 10084'U at the embark, during my testing)
113 (10083'U) no wild animal wandering. insects/vermin appear. ***** hottest insect temp. from here and hotter, surface stockpile drink consumption ok, food not ok.
112 (10082'U) no wild animal wandering. insects/vermin appear. surface stockpiles..both food and drink consumed as expected.
110 (10081'U) no wild animal wandering. insects/vermin appear.
106 (10078'U) no wild animal wandering. insects/vermin appear.
105 (10077'U) no wild animal wandering. insects/vermin appear.
104 (10076'U) wild animals appear as normal. insects/vermin too. ***** hottest wild animal temp.
103 (10075'U) wild animals appear as normal. insects/vermin too.
100 (10073'U) wild animals appear as normal. insects/vermin too.