For starters, tanks are tanking just fine. Flat pen no crit ADCs are not murdering tanks. The only way an ADC will melt a tank is with 3 multiplicative scaling stats: AS/Crir/percentage armor pen and they need ALL OF THE ABOVE to kill tanks. They spent the first 3 items of their builds actually beg unable to kill tanks. Until they pick up LW > LD and AT LEAST some attack speed, they will tickle tanks.
Here's what's actually happening:
Tank mains or going way too ham, jumping into the enemy teams going on their backlines and between being kited/cc'd; they die in 3-4 seconds when being focused by AN ENTIRE TEAM. They are mad that their full tank no damage build isn't murdering squishies anymore and as a result, they also think they should be able to be 5 man focused and live forever. No.
Tanks weren't not balanced during cinderhulk meta and tanks were not balanced during this season's tank meta. Tanks jobs are to zone control, damage soak and CC. If they pin down priority targets, that's well and good and that means they've done their job incredibly well. What is not their job is to Zerg back lines and KILL adcs and mages. When that's happening, tanks aren't balanced.
Stop QQing, tanks spend the first 30 minutes of the game being unkillable. There doesn't need to be an item that reduces damage taken from a percentage standpoints, offense should ultimately beat out defense in a game like this. A champion spends its entire build at the cost of maybe 1 defensive item to kill tanks. Instead of QQing that tanks can't do everything, play the role or find something else if you think a full tank building killing squishies whole ignoring everything thrown at it is balanced. It's not reasonable and nobody who isn't playing tanks thinks it's reasonable.