A 20th-level character is super-human. This follows from the fact that a 6th-level character is super-human. A 6th-level fighter, with full HP, faced with a mundane heavy crossbow, will dodge it almost every single time, barring a low constitution score, low HP rolls (if HP rolls are used on level-up), and/or a critical hit combined with a high damage roll. A 20th-level fighter might pluck the bolt out of where it's barely dented their bare chest, and look amused. This is derivable from the fact that a character is not impaired by hit point damage until they hit 0 (zero) HP.
The logical take-away is that any HP loss taken before then, does not represent severe injury, but, as noted in various rulebooks over the years: fatigue, use of luck, minor cuts and bruises, and the wearing away of protection granted by their deity.
With a 10 constitution and exactly average rolls (5.5), a 6th-level fighter will have 33 Hit Points. Facing a character with a 10 strength wielding a longsword (1d8 damage), they can then evade, reduce, be glad it was a close shave, and be saved by some divine act, an average of 7 and 1/3rd times that sword would have killed them.
The take-away is not, generally speaking, that they are ignoring a gaping flesh wound. Although that is appropriate for some creatures, such as zombies.
This should be required reading before discussing or critiquing D&D 3.0e, especially it's power curve or HP:
https://thealexandrian.net/wordpress...expectations-2