Thornmail with Grievous Wounds is at odds with itself.

TL;DR: Put GW on Randuin's Omen, delete Thornmail.

Assume the adc's auto attacks deal 100 damage

Thornmail would then reflect 15 damage per auto attack

0% lifesteal: -15 hp per auto attack vs. 0 hp against non-Thornmail targets, devastating to a careless adc who doesn't realize the tank has bought Thornmail. 20% lifesteal: +5 hp per auto attack vs. +20 hp. Reducing lifesteal healing by 75% is a big penalty for the adc, but they aren't killing themselves anymore and even still see some positive regen. 40% lifesteal: +25 hp per auto attack vs. +40 hp. Not even a 50% reduction in lifesteal healing means the adc stops really feeling the passive of Thornmail, the enemy tank is still an HP battery.

Thornmail's damage return passive makes lifesteal a higher priority on adcs, not a lower one, so they can work to make the damage return into an insignificant portion of the damage calculation. Grievous Wounds as a mechanic is obviously meant to directly attack the healing from lifesteal to make this item into a counter for large amounts of lifesteal.

Tank players may be thinking "Well that's great, now I can build this item and it's just always good against adcs!" But power in this game that is "always good" generally doesn't stay that way for long. If it really would be strong enough to be "always" good then it's a must-build every game that screws over an entire class of champions therefore it's OP and would need a nerf. Since it's an item that is generically useful every game, if it's nerfed to not be a must-build item, then it's now a never-build item since there would always be a specialized item that fits the scenario better.

I propose a different tank item for GW: If the goal of this change is to create an item that allows a tank to keep themselves from being an hp battery to a fed, lifestealing adc, then I think a better choice would be to put GW on an item that builds out of Warden's mail and also possesses the 'Cold Steel' passive AKA 'enemies that auto attack me get an attack speed slow'. This would reduce their lifesteal directly and also hamper their damage output to further reduce how much they regen, creating an item solidly in the well-needed niche of "stop the fed Caitlyn from staying full health when all she did was right-click me and then afk until the fight was over"

This still leaves open the question of, "How do we make Thornmail relevant?" To which I don't have an answer. I am personally of the opinion that the damage reflect passive and the fantasy it creates is toxic and irredeemable. If Thornmail were to be made relevant I think its passive would have to be re-imagined as something completely different. If a tank wanted to punish someone for attacking them, then maybe something like "Enemies that auto attack you are marked for 3 seconds. Activate this item to fire a burst of thorns at all marked targets, dealing 15% bonus hp magic damage and slowing the target's movement speed by 50% for 3 seconds, 80 second cooldown" which would make it an effective anti-kiting/skirmishing tool since an adc would be very frightened of trying to poke you with an auto attack unless their full team is around to cover for them if they eat a slow.