Look: it is insanely unusual to find runway YSL or Dior in a thrift store. Of the hundreds of people who post in this thread, it's happened a couple of times. I've seen one piece of either in my life, and I've seen many pieces of Brioni, Kiton, Chester Barrie, Savile Row stuff, whatever.
What you see lots of is the licensed stuff, and the licensed stuff from the 60s through the 90s and early 2000s is crap. The only reason someone would buy it is for the brand name, thinking it was good, and buying based on name. Which is why they licensed the name, made a bunch of money, and almost ran their brands into the ground.
The odds of you spotting Dior Homme or YSL Rive Gauche in a thrift are one in a bajillion. So if you don't know the difference, it's safe to assume that if you see YSL or Dior, it's crap. Stuff from Macy's and JC Penney's in the 70s and 80s. Not something worth buying used at thrift store prices, much less bragging about.
The one Rive Gauche coat I found, which was from the Tom Ford era, was stunning. It was very, very obviously not licensed junk. It was clear. It was too small for me, but it went for a few hundred dollars on eBay.
Frankly, I think there are a lot of folks in this thread who could really up their thrifting game by focusing on identifying quality and wearability rather than reading brand labels.
So, I found a YSL Rive Gauche DB suit. It seems nice. It seems like it will fit well with some alterations. I'm still learning to identify quality and bought this not knowing anything about Rive Gauche at all. I more use labels to tell me when to leave things on the rack. So, do I have a one in a bajillion find?