Thornmail sucks balls for most champions
Besides
who can only make it useful with having obnoxious amounts of armor from their kits and builds.
is a very very bad item to build, either revert the nerfs or rework it to be sort of like that dota 2 item that when activated reflects damage back to the enemy.
Example of how it could be reworked:

Same cost.
75 armor
Unique Active: For the next 0.5 second(s)(+1 second every 50 bonus armor excluding Thornmails) you inflict 50% damage received from physical auto attacks and physical spells/abilities to the champion that hurt you as physical, magic, or true damage(45s cooldown).
Made like this so it isnt a rush or 1 item wonder for adcs to abuse to hell. A tank/bruiser build of
. With that build the total bonus armor(excluding Thornmail) including armor yellows is 60+9+50+50+30=199 bonus armor(let's say 200) gives the player a 6.5 second active.
If an ADC built thornmail and ninja tabis they would maybe have a 1.2 second active.
If a mage went thornmail, tabis, and zhonyas they would barely go over a 2 second duration.