There was a time when big stores (
walmart, Target, etc.) weren't allowed to sell most of those specialty products. Back then you could call the company (Bed head, Redken, etc.) and they would fine the store for carrying the product they weren't supposed to have. Then every one of those stores that opened a salon was allowed to sell, and as far
as I can tell that industry standard went away because it was too much hassle and nobody wants to know their competitors products are being sold while theirs are not.
Yes, I think the industry standard has changed. It used to be that the those products that only sold in salons were considered more high end, better products,
worth making a special trip to the salon for. But, there is no way that Paul Mitchell, which is now a multi-billion(?) dollar company was able to make all those billions selling through salons only. There just aren't enough salons, nor customers there. There is just no way he could stay competitive with companies like Pantene and
only sell in salons, when there are drugstores every few miles and thousands more customers there who could pick up his products just as easily as a Pantene bottle
and be willing to pay a
few dollars more for his bottles.
Fashion designers do that too, now. I have forgotten who was the first high couture fashion designer who started selling a cheaper line in a chain store. Other high-end designers thought it was ghastly that that designer cheapened their own brand name, by
lowering themselves to mall people.
However, all the other designers watched & waited. And for all their snootiness, when they realized that designer was raking in bucks for their mass-produced/chain store line, and that in fact, made that designer's name even more famous among the lowly masses, many, many designers now are doing the same thing. They very carefully differentiate their couture line from their mass-produced line.
Michael Kors has a couture line and his chain store line that he sells at Macy's. Issac Mizrahi sells at or used to sell at Target and now sells on QVC or HSN. Vera Wang sells a non-bridal line at Kohls.
As for whether the drugstore bottles of the salon products are close to expiration, there are many articles out and research done that the expiration dates on most items are just arbitrary, if they not perishable food products. Even with the perishable products, they set an expiration date long before it will actually go bad, as a precaution,
knowing that many people will use the product right up to or even after the expiration date. Do people actually think a product goes bad right on the date on the product?
Even milk, depending on the state it is distributed & sold in, can have differing expiration dates from a neighboring state. Same cows, same farm, same distributor can have two different "sell by" dates depending on where the end product ends up.
It would actually be quite smart for Paul Mitchell to stamp a quicker expiration date on the same exact batch of shampoo that goes into the big box stores, to
pretend that the big box stores have "soon-to-be-expiring" deep-discounted shampoos.

They have probably done extensive research on customer psychology on what is a good "expiration date" to put on the ones sold in the big box stores and still have the products sell at a good pace so there are not too many left over bottles still on the store shelves after the arbitrary expiration dates that then
do have to be needlessly dumped & wasted.
And yes, there are retailers who will take the truly "soon-to-be-expiring" overstock products that never made it to salons. Or they will take the discontinued products. I have a Big Lots type of store near me that routinely sells discontinued beauty products. I love the store. When I find a product I like selling really cheap, I have to go home and test it out right away. If I love it, I go back to the store and buy several more bottles before they sell out. I know I can't get too attached to that product, as once I've used up my stash, that is it for it.
I have gotten many Got2Be and Ponds products that way. Sometimes the marketing of a certain item, or the packaging just doesn't move the item, so they discontinue it or re-package it for the higher retail price and deeply discount the non-moving, discontinued items in Big Lots type stores.