It's not the drain tanks that are broken, it's Conqueror

The simple breakdown is: Old conqueror healed for 8% damage while the current one heals for 15%. Keep in mind this is essentially 15% spellvamp and lifesteal.

Now, the difference between the two besides amount is that old conqueror healed for pre-mitigation damage while new heals for post. That sounds fair, right? Old conqueror would be better later since it ignores the acquired defensive stats enemies would have for healing, and new would just be better earlier since that's when resistances haven't been built up. All that'll do is shift the timing when conq's the best. Shouldn't be too much of a problem, right?

Problem arises when MR on a lot of champs doesn't scale up, and the ones it does scale on MR scales very slow - that's why Morde saw a win spike after his nerfs going into pre-season. Not to mention that 15% mitigated healing turns to unmitigated healing when that damage is straight up true damage - that's why Fiora is suddenly busted without receiving quite literally any buffs since well before last season ended. You even have Lee Sin running Conq instead of Electrocute now because of how fast he can stack Conq, and once it's stacked its benefits far outweigh pretty much any other rune.

And as for laning, with tanks historically having a very mediocre laning phase when paired against bruisers, the shift of Conqueror being stronger early makes it so that in many cases, the champions that would be reducing Conqueror's effectiveness flat out don't ever get do do that and so aren't even hardly seen - so in most cases, new Conqueror will straight up outperform old Conqueror across all stages.

Conqueror either needs a rework or a revert right now due to how egregious the healing on it is and how stifling it can be early.