so... when are we removing the DR on PD?

no ... it was and still is a dueling item. It only stops damage if you have attacked them first. If the adc is attacking the assassin before the assassin has started their combo then they already failed their burst.

It was never meant to protect ADC's from assassin. Very few marksman even bother to build PD. This damage reduction doesn't give much when you are squishy AF it begin with.

Basically only 3 champions who build it on regular basis are Vayne, Yasuo and Graves. All of them are meant to be strong in 1v1 (whether or not it's a good thing is a different matter). It's not like PD is going to save i.e. Sivir from Zed attack.