When diagnosing a problem, one considers the "rollback". Going back to a point in time where everything was known to work, and changes made since then that could have caused issues reverted.
What should have been done months ago was a straight revert to the point where Sejuani was the queen of the rift for exactly one patch, because nothing else since then has made a single lick of sense. When Sejuani was at 60% winrate, tanks were the ones building tank items and most players agree that diversity was at an all time high. (And then you can skip dumpstering Sejuani and instead address Cinderhulk properly, but this would be Riot admitting they fucked up which they never do. Joking about Zed in patch notes where Zed is not nerfed does not count.)
Riot is now nerfing tank items instead of addressing the core issues of class balance being completely out of whack. People are buying tank items on everything because they need to in order to use an entire rotation before dying. Nerfing the tank items isn't going to fix that, it's just going to shove out even more champs. You know the wheels have fallen off when Sterakk's Gage gets essentially deleted due to its marksman abuse instead of just made melee only, which would get it back on the classes it was intended for (good) instead of ruining it for everyone (stupid).