I looked it up and apparently it was a poison touch passive. So yeah, somewhat like Thornmail because it activates when the tank is attacked, but like immolation in that it dealt damage over time.
Smiting the Gromp grants Gift of the Toadstool - attackers are poisoned for 6 (+ 6 per level) magic damage over 3 seconds.
They patched it a few times.
DAMAGE SCALING : Gift of the Toadstool'd champions deal 6 + (6 x level) magic damage over 3 seconds ⇒ 4 + (8 x level) magic damage over 3 seconds
They decided tanks weren't popular enough so instead of giving them some better tank scaling usage out of items, over-buffed this Smite Buff.
GROMP POISON : 4 + (8 x level) ⇒ 10 + 10% of bonus health
They realized they went too far, so they tried to tone it down.
DAMAGE SCALING : 10% bonus health ⇒ 5% bonus health
But the problem was no longer about the numbers but about how tanks were getting power. It made the jungler slot dominated by tanks and that's what made people annoyed. So they took it out. People talk about Cinderhulk as if it was ever a good damage item. It's not. Gromp's Poison is the true reason tanks were dominating the jungle. If you tie tank damage to only HP, you have to be conservative about it.
That's why champions like Braum, Nautilus, and Taric don't have too much HP scaling...it makes them tanky against both magic damage and physical damage. Even if you bring that interaction back, tanks can just ask for a ban on
for fear of them buying
right? So the better solution IMO would be to tie either armor or MR resist to such an interaction. And it could go on the new jungle item, since the AP jungle item scales with AP anyway, it's only fair that tanks get something to scale with better.
Tanks that go all in on an armor jungle item would lack HP and lack MR, so mages could still bring them down with ease. Tanks that go all in on a MR jungle item would lack HP and lack armor, so AD champs could still bring them down with ease. It wouldn't solve all of a tank's problems, but it would give them options, and if the passives for those jungle items are done right, then tanks could become a competitive but not dominating choice in the jungle for all players.