I volunteer in one.
My take on it is:
We still get some amazing quality donations. I priced a bag on Thursday which was all Boden, Phase Eight, Hobbs, Reiss, Cos.
Those bags have become less common in the 8 years or so I have been volunteering.
Perhaps some people are selling on Vinted, many of the bags we have handed in though are from people in the 40+ demographic and maybe they aren't as interested in Vinted/Ebay? We are located in a fairly affluent area, they may well think the £ they can get from selling their clothes isn't worth the hassle.
Fast fashion has had a FAR larger impact. Things from Primark, Shein, Pretty Little Thing, Boohoo, H&M - often far too poor condition to even put out on sale, those sorts of brands rarely even hit the rails unless BNWT or something more expensive like a winter coat. Nearly all of it straight to recycling.
A large % of our turnover used to be occasionwear - wedding guest dresses, mother of the bride outfits, prom dresses, ball gowns, hats etc. This is the first year that occasions have been back to normal, we've very little supply of that sort of item.
We in our chain do get corporate donations from large brands, stuff which is mainly online returns, sometimes ex-sale stock, sometimes minor faults like a split seam or missing button. Spent most of Thursday pricing boxes of Zara stock - very little of it to my taste as a 50 year old woman but my teenage DD thinks it's amazing to get a Zara dress for £10 which would have been £30 in the shops. That's our rule of thumb for new - a third of the original selling price. We also have loads and loads of swimwear in at the moment, not Zara, but I think it's stuff which was ordered in for summer 20 and 21 and then nobody bought it as we couldn't travel.
So in short - yes, still bargains to be had but the key is to be checking regularly.