A brief history of what Shuttle crews wore for ascent and entry, from the Columbia Crew Survival Investigation Report:
The shuttle was originally designed to be operated in a shirtsleeve (bare-hands) environment for all phases of flight, including launch and landing. The U.S. Air Force SR-71 pressure suit was worn for the first four shuttle missions (STS-1 through STS-4), which were considered test flights. Following these first four missions, the shuttle was declared fully operational and shuttle crews wore standard flight suits (lightweight fabric coveralls) with a helmet and a portable air supply that was intended principally for ground egress and cabin smoke protection. After the Challenger accident in January 1986, NASA started to use the launch and entry suit, beginning with STS-26 in 1988, and later phased in the more capable ACES beginning in 1994.
The report notes "Shuttle crew members are surrounded by layers of protection, with the crew equipment being the closest layer, the CM [Crew Module] being the next layer, and the vehicle being the outermost layer." Advanced Crew Escape Suit (ACES) was a further evolution of the SR-71 suit intended to provide extra protection and survivability at the "Crew Equipment" layer, if the crew were to lose the protection of the outer layers — a possibility that loomed larger post-Challenger. To this end, ACES was bright orange for Search And Rescue visibility; could be fully pressurized, with personal emergency oxygen supply sufficient to reach the ground from altitudes above 100,000 feet; was "certified to protect a crew member exposed to an atmospheric/altitude environment up to 100,000 feet" (so O2 supply was not the limiting factor); had a flame-resistant Nomex outer covering; and came equipped with parachute, life preserver unit (i.e. personal flotation device), seawater activated release system, inflatable life raft, satellite-aided tracking beacon, AN/PRC-112 radio, emergency water pack, survival mittens, smoke illumination flare, emergency signal mirror, light sticks and a few other bits of kit... no wonder it was bulky! Pretty much the opposite of the sleek SpaceX suit.
Despite these features, and even though a substantially superior suit wouldn't be expected to save any lives on Columbia, the Survival Investigation Report was critical of ACES. "Five events with lethal potential" were identified, numbered (the "X" in "Recommendation LX-Y") as 1. depressurization of crew module, 2. lethal trauma from dynamic rotating load with lack of upper body restraint and nonconformal helmets, 3. separation of crew from module and associated forces/heat, 4. exposure to near vacuum/high acceleration/cold, 5. ground impact. The suits could or should have protected against most of these, though point 3 "was not survivable by any means currently known to the investigative team". The report noted how in 1966 an SR-71 pilot survived break-up at "Mach 3 at over 75,000 feet (~400 knots equivalent airspeed (KEAS))", partly due to the pilot's closely-related suit automatically pressurizing. In contrast, no Columbia crew were able to manually pressurize their suits in time, which required lowering and locking visors. Due to pre-entry tasks, three of the seven had not fitted their gloves, with one not yet having donned their helmet, so they could not have pressurized their suits anyway. The fact helmets were not designed to conform internally to the shape of the head contributed (alongside harness failures) to lethal injuries during rotational motion. During separation, the suits failed mechanically and came apart from the crew's bodies, despite the dynamic pressure being well within design limits and similar to the 1966 SR-71 incident. The report attributes this to visors being up, with some speculation about thermal conditions or high-altitude chemical conditions e.g. monatomic oxygen. Some components had considerable heat/fire damage, e.g. where parachute harness straps and the off-the-shelf boots used nylon (in contrast to a U.S. Air Force suit). Parachutes would only have opened manually since the crew did not exit using the crew escape pole (necessary to initiate the automatic opening sequence), so could not have protected unconscious crew from ground impact even if the suits had maintained integrity. Recommendations for future American launch/entry suits were:
Recommendation L1-2. Future spacecraft and crew survival systems should be designed such that the equipment and procedures provided to protect the crew in emergency situations are compatible with nominal operations. Future spacecraft vehicles, equipment, and mission timelines should be designed such that a suited crew member can perform all operations without compromising the configuration of the survival suit during critical phases of flight [i.e. tasks to be performed immediately before entry shouldn't have required gloves/helmet off].
Recommendation L1-3/L5-1. Future spacecraft crew survival systems should not rely on manual activation [referring to suit pressurization/parachute] to protect the crew.
Recommendation L1-4. Future suit design should incorporate the ability for crew members to communicate visors-down without relying on spacecraft power.
Recommendation L2-4/L3-4. Future spacecraft suits and seat restraints should use state-of-the-art technology in an integrated solution to minimize crew injury and maximize crew survival in offnominal acceleration environments.
Recommendation L2-7. Design suit helmets with head protection as a functional requirement, not just as a portion of the pressure garment. Suits should incorporate conformal helmets with head and neck restraint devices, similar to helmet/head restraint techniques used in professional automobile racing.
Recommendation L3-5/L4-1. Evaluate crew survival suits as an integrated system that includes boots, helmet, and other elements to determine the weak points, such as thermal, pressure, windblast, or chemical exposure. Once identified, alternatives should be explored to strengthen the weak areas. Materials with low resistance to chemicals, heat, and flames should not be used on equipment that is intended to protect the wearer from such hostile environments.
These recommendations point towards a kind of "Super-ACES", offering even greater protection. Not much seems known about SpaceX's suits but their design philosophy looks like it's moved in the opposite direction from ACES — presumably because they are emphasising survivability at the crew capsule layer rather than expecting suits to handle so many scenarios. To what extent have the above recommendations been implemented, or even required by NASA's specifications? Have the parts about protecting an astronaut falling through the atmosphere after a break-up (windblast protection, automatic parachute) been ignored entirely? Regarding L1-2, the new suit appears designed for ease of use, but is there a compromise from having an external oxygen source? Do we know much about the helmet design and how it interacts with other head/neck protection from the seats, or communication if the craft loses power? And how stringent are heat/fire and other resistance requirements compared to ACES?
Somewhat related question: Are SpaceX new space suits inferior to existing ones in terms of usability?