What is the maximum indoor temperature beyond which you can't work?

40+ C in Saudi or Australia. 34 or so in Malaysia.

Humidity matters a lot. Get a dehumidifier. The cheapest electrical dehumidifiers are air-conditioning.

Depends on the work. If I'm sitting at a computer and not particularly doing anything physical, 35c to 40c isnt bad. Perhaps even enjoyable.

If i'm lifting equipment, moving desks, that sort of thing... 20c, at 25c im going to be soaked from sweat.

If im working with hot work, like welding or some foundry or greenhouse, probably same idea around 20c. 25c will be too hot to deal with it.

What matters is https://en.wikipedia.org/wiki/Wet-bulb_temperature

If I've acclimated - a very dry, breezy mid-30's (C) or mid-90's (F) is still workable, if not optimal, for work with minimal physical exertion.

30 Celsius, but as others write it's the humidity that does the trick. I can do 40 C and super-dry, or 22 and super-humid (and everything below the line that connects these two points).

78 degrees Fahrenheit.

Around 28 celsius is already very bad, but a fan helps a lot. And cold water.

Are you asking for the safety regulations of each region and line of work or peoples individual limits?

23C/73.4F