I find it a bit strange that the plates just suddenly disappear at a given time. It seems a bit illogical, kind of like some other unintuitive changes for outer turrets now or the situationally reduced gold from junglers when farming lanes pre-jg item etc. I think the game should be easy to grasp and internally consistent. These types of "gotchas" and internally inconsistent changes do have logic but they make the game less friendly and more opaque (especially to newer players). Ideally, there should be a minimum of exceptions to rules and game logic.
For turret plating, I don't like the arbitrary removal at a certain time. It violates expectations and seems random. Ask yourself if a new player would expect this. It feels out of the blue so while I like the idea behind it, maybe there are other implementations.
Perhaps cannon minions get some sort of new armor piercing weapon that visually does a ton of damage to the plates now. Maybe melees get battering rams.
Another idea is to have plates wear down with time, exposing cracks and slowly falling apart. Even if you want them to eventually self-destruct, these sorts of visual and gameplay indicators seem more natural. This would of course be paired with an increase in the damage they take at that point in the game. For instance, perhaps they become permanently 10% more vulnerable to magic and physical damage every 3 minutes. Of course this would be paired with the visual effects showing them losing structural integrity. This would obviously require testing to make sure they don't become just "gravy" you save for later or have your team keep to offer it to the ADC etc.
I haven't given it too much thought but I'm sure there has to be a better and more natural way of integrating this idea.