True, but right now it is a sh-t tier item, it needs a buff in general, because right now it is not useful, you just feel you are buying 100 raw armor because the passive sucks unless u are Rammus, and still...
With doing math on Thornmail I'm pretty sure it's issue is more in the perception vs. reality realm than anything else.
Everyone expects it to net you AFK kills but it doesn't.
Not because Thornmail is weak, but because that would be stupidly OP if it actually did net AFK kills.
It's worth noting that Thornmail reflects damage taken prior to damage taken being mitigated by armor, %damage reduction, damage absorption, blinds, dodge, or anything else that reduces AA damage. As 100 armor means 50% damage reduction, 1/2 damage taken, or x2 defense: you're generally reflecting a pretty substantial % of the damage directed your way, much more than Thornmail's tooltip lets on.
Also: with Thornmail's current incarnation, it's objectively better than when it reflected 30% damage. Comparing realistic armor and realistic AD values, it hasn't shifted it's return damage in the face of champions like Riven that just build pure AD at all.
It has however gotten worse at countering critical-hits and sheen procs, but better at dealing with on-hit effects. OK vs. a wide range of champions due to offering a decent response to on-hit effects makes thornmail a better item overall over just punishing crit-stackers, as it brings something fairly useful against a wide range of top-lane picks: as well as fairly consistant reflected damage vs. certain crit and on-hit champions like Vayne.
Furthermore: it's worth considering what's a realistic gold value of Thonrmail's passive and what it can accomplish in that space over being disappointed it doesn't net AFK kills.
Thornmail's passive costs 350 gold based on Chain Vest's pricetag.
In that space: Thornmail shuts down any life-steal used against it completely. Due in large part from reflecting damage prior to that reflected damage being mitigated, it's extremely difficult to amass the amount of life-steal necessary to out-heal thornmail, and even when you do: you can't really notice any benefit from life steal in that case. Life steal negates thornmail and thornmail negates life-steal, removing thorns and life steal from the equation on both parties.
Thornmail also does offer a lot of power vs. jungle monsters, where you'll reflect over 40% of their damage back to them considering just Thornmail and base armor plus at least 25 additional magic damage from the armor-to-damage portion. Pretty handy vs. hard-hitting mobs like Dragon and Baron, especially later in the game if you're trying to sneak one of those out from under the enemy team while other members run distraction.
And Thornmail makes you fairly unappealing to focus, which can be useful for a tank that want's some weight to push around with footsies and zone-control, or a semi-squishy that would like to make an opponent think twice about focusing them.
I'd say all that is worth at least 350 gold.
True, but right now it is a sh-t tier item,
It's more niche than anything else. Thornmail is an OK item at the moment that brings a respectable level of power in the right circumstances. It's still purchased every now and then at the pro level, often on duel-happy top-lanners trying to push an advantage.
I'd reserve sh-t tier for items that are just a huge gold sink and do nothing useful.
With thornmail: you're only down 350 gold if the passive isn't doing the right job, which still allows the item to be useful if the situation really called for armor.
Again: more a perception vs. reality thing. Thornmail objectivly isn't that bad, it's just its ~350 gold passive seems to indicate it can do things it simply cannot offer with a ~350 gold passive.