I was going to make this a comment, but it became more involved…
What you are calling “impossible” is merely “difficult for the situation”
In the comments you gave an example:
If a creature’s AC makes it impossible for you to hit it with all mods and bonuses but a natural twenty always hits I feel like the natural twenty in this case only grants you the hit not the crit since you are only able to hit it because a nat twenty always hits. Example: a first level fighter with a 16 strength using a long sword comes upon a creature with a 25 AC (not likely but bear with me) the only way for the fighter to score a hit is a twenty +3 for strength +2 for Proficiency = 25. so its a crit every hit seems odd
But that’s not an “impossible” AC to hit. It is only very hard.
If that fighter also had a Bless spell on them, it would give an additional d4 meaning they could potentially hit on a 16! So long as they also rolled a 4 on the Bless die roll. Or someone gave them a enchanted (+1, +2, or +3) weapon. Or they were using a bow, had a 16 Dex, and had the “Archery” fighting style for a +2 bonus.
It just so happens that for a given scenario, to hit a certain AC, it requires a 20 plus all of a players’ current bonuses/mods is just math. The crit is still a crit.