It is stated in the 1.16 patch notes (see attached image) that knockback resistance is not luck-based, but a scale now. This doesn't work correctly however.

To test, summon a zombie, use the following command:

/attribute @e[type=minecraft:zombie,limit=1,sort=nearest] minecraft:generic.attack_knockback base set 5

And get hit by it a few times to see how much knockback it deals.

Then, either equip Netherite armour or use the following command:

/attribute @s generic.knockback_resistance base set 0.5

You'll find that while the knockback taken is reduced by roughly the correct percentage, there is also a probability of taking 0 knockback.