I suggest that, instead of having an 80% offhand proc, the nerf should be -x/-y damage for using TWF. For example, -4/-2 at TWF and -2/-0 at GTWF. The devs can adjust them accordingly as they see fit, they could calculate the break even point at which -x/-y damage is equivalent to the damage decrease from 80% offhand proc.

This reflects the weakness of the TWF combat style, you cannot put the full power of both arms behind either an attack or block. Also, TWF is much harder to handle (already reflected by the -x/-y of the attack bonus).

Attack bonus is an indicator of a character's skill with weapons, while AC is an indicator of a character's defensive prowess. As characters level up, their base attack bonus increases, reflecting their increase in combat experience, and hence increase in skill handling their weapons.

A Level 20 character should not be missing on a CR1 Zombie, thats just silly. A significantly more skilled character will always be hitting his opponent, he will create openings and take advantage of the holes in the opponent's defenses. He should not be missing his opponent due to pure chance (80% offhand proc).

He will however, suffer the same lack of power (as compared to THF) behind his attacks, regardless of whether he is hitting a CR1 Zombie, or CR20+++ Pit Fiend.

Gameplay-wise, its not fun for your character to be missing due to pure chance. People dont like missing due to the RNG handing them some bad rolls. It would be kind of like a 50% resistance to haste. Hence my suggestion for 100% offhand proc, but a -x/-y to damage. This removes the randomness from the system, and imo, will make the nerf far more palatable.