Commercial jet airliners fly in the stratosphere, but because they are pressurized, they could be said to have a shirt-sleeve environment.
Crews of the US Apollo spacecraft always began the flight phases of launch, docking, and re-entry in space suits, although they could remove them for many hours.
This worked well, until an accidental depressurization on entry resulted in the deaths of an entire Soyuz crew.
This term is also used in science fiction to describe an alien planet with an atmosphere breathable by humans without special equipment.
[1] The Space Shuttle's Spacelab Habitable module was an area with expanded volume for astronauts to work in a shirt sleeve environment and had space for equipment racks and related support equipment for operations in Low Earth orbit.