From the living stone bridges rotting while they're still live, to the darkness pushing in on the light, or even just... The Aes Sedai straight up saying that they could feel the taint of the place, and how it would taint anything they tried to do there with the OP... Which mind, we also saw actually happen in the EotW.

As far as the cleansing though, that isn't how the taint works. The cleansing got rid of the taint on Saidin. The taint already in the ways would of stayed right where it was, much the same way as male channelers weren't just suddenly cured of the madness they already had. The ways wouldn't get any more tainted, true... But they're over three thousand years old. They're already pretty much as tainted as they were ever going to be.