In the patch notes on the Minecraft wiki, it claims that knockback resistance was changed to reduce knockback by a scale amount of the resistance provided as opposed to being a probability to take no knockback.

Source: https://minecraft.gamepedia.com/Java_Edition_1.16

However, if you test knockback resistance, it still behaves the same way that it used to.

Steps to reproduce:

1. Open up any world and type the following command:

/attribute @s minecraft:generic.knockback_resistance base set 0.25

1. Summon a hostile mob and let it attack you in survival or adventure mode.
2. Observe that the player will fully ignore knockback 25% of the time.