The irony is that (at least I'm pretty sure) the concept of "room temperature" was originally pinned to 70° Freedom Degrees. Your 21°C reference unit is probably a rounded conversion, which explains the sibling comment arguing 23°-25°C as potentially inferior alternatives. An accurate conversion would be 21.1̅°C.

Conceivably, if you had an umbrella to protect yourself from the acid rain, an oxygen mask, and were on a very safe floating platform in the upper atmosphere, you could walk around in a t-shirt and be just fine.