I've been ATGATT up to 105F (40.5C) once, and very close to that (98-99F or 36-37C and humid at 95%) several times as well. Each time was on my ride home from work so I was Business Casual dress at work and did not remove any of my work clothes for the ride home other than leaving my work shoes in my cubical in favor of wearing my mc boots for the ride home.
My riding gear is multi season and has removable layers or liners. Both my armored riding jacket and overpants start as a mesh base. To the jacket I could add a solid layer over the mesh, then as it became cooler I could add a wind liner inside and then an insulation liner inside that. The pants were basically the same.
When I experienced the hottest rides home I would put on my riding gear at my cubical in mesh mode and as I walked to the front door of my building the cool air of the AC made it seem like I turned on my own personal AC inside my gear as I walked. Just from the air moving through the mesh. The design and materials of my gear made that possible. As soon as I exited the building the outside heat made me start to sweat. The cooling from the walk inside stayed with me just a little but then was replaced by the hot air. This was all before I got to my bike. Then as I started to ride through the streets evaporative cooling took over and I was comfortable. My ride home was at least 13 miles and more usually 26 miles (longer going home since I wasn't on a time restraint). I didn't care if my clothes got sweaty and stinky during the ride home since I always changed out of them anyway as soon as I got there and they would be put in the wash. Since evaporation dried out my clothes rather quickly with my jacket being full mesh instead of simply mesh vents I was in a constant cycle of sweat/evaporate/sweat/evaporate... which did end up keeping me cool enough to be tolerably comfortable enough to enjoy the ride anyway. (I can still tolerate some excess heat or cold at 68). I also wear a full face helmet that has good ventilation and I wear Tour Master Air type boots and summer gloves with a good knuckle protection (AlpineStars Copper gloves).
My current jacket does not have the full mesh base. The armored over pants still do though. My current jacket has very large mesh vents on the chest, biceps, and a very large vent that is more than half of the back. I can adjust these to be full open or partially open or closed. I also still have the inner liners to put in for cooler or colder weather. I have the Tour Master Pivot jacket.
With this gear I can still ride in temps that range from the above mentioned high temps down to around -10F or just under (-23C or just under) depending on vents open/closed or what layers I put in or add. At the colder end of the temp range I put on 2 pair of socks with the same boots (scooters have very good foot weather protection) and I have winter gloves that fit into my Kemimoto handlebar muffs on heated grips to keep those bits warm.
I mentioned on another forum that when I was working I was either riding my bike to work or taking the bus (our bus system is free for military vets... me, and picked up on block from my home and dropped off across the street from my work). The only time I really took the bus was if it was snowing or if there was still snow/ice on the road. Otherwise it was riding the bike to work. Rain was/is not a deterrent. I wear Frogg Toggs rain gear over everything for that. I like keeping my riding gear dry. Much more comfortable that way than relying on inner rain liners that soak the primary gear before they stop the rain from passing through. BTDT and didn't like the clammy feeling I got with that.
I got really good at watching and interpreting the weather reports. Notice how the weather forecasters didn't get it right all the time?
So far I am able to "stick" with ATGATT every time I ride... The only exception might be the one tenth of a mile around the cul-de-sac on my street while testing some maintenance performed, which is very rare.
