NASA used to use a cabin pressure of 5 psi with 100% O2 as the space capsule could be built much lighter. Astronauts would pre breathe O2 for several hours prior to flight to prevent decompression illness. Basically the same thing that is done now before EVA's. 100% O2 atmospheres were however very dangerous on the ground at 14.7 psi and after the Apollo 1 fire, NASA modified their approach slightly to use a normal nitrogen oxygen atmosphere at launch that would then vent and be replaced with 100% O2 as the craft ascended.
The Soviet's had better booster technology and could loft hevaier payloads so they stayed with normal atmospheres at approx 15 psi. However they lost 3 cosmonauts during the rentry and descent of the Soyuz 11 when a valve opened prematurely and vented the cabin pressure. Both the Soyuz and Voskhod capsules could carry 3 consmonauts but only if they left their pressure suits behind, so the missions with 3 crewmwmbers were flown in what amounted to sweat suits and they were consequently not protected from decompression during the flight.
The vacuum in space is a great insulator but the temperature of an object in space relatively close to the sun depends on whether it is shade or in sun light. The shady side of an astronaut on an EVA will be a couple hundred below zero degrees while the sunny side will be a couple hundred above zero.
As indicated above, high altitude flight above 55,000 ft requires a pressure suit. But even at lower altitudes down to approx 40,000 feet, breathing is problematic as even with 100% O2 delivered in a positive pressure mask, the PO2 is so low that transfer of O2 in the lungs is minimal. So the issue at high altitudes is not just one of having to worry about nitrogen bubbles but also having to worry about excessively low PO2 even with 100% O2 that results in hypoxia and eventually anoxia.
