I have recently stopped volunteering in a nice shop in a decent area. We still got OK clothing donations as not everyone can be bothered with Vinted/Ebay. But yes, over the past 10 years the quantity of clothing donated has not declined but the quality definitely has. Nobody wants shit from SHein or Temu second hand.
The bottom has also collapsed from the rag market - remember 10 years ago there were all those "cash for clothes" portakabins everywhere? You don't see them any more because the rag market is absolutely flooded with cheap shite and there is no profit in it any more. Charities get around 75p per black bag full of textiles, if that. Rising fuel costs makes it even harder to get rag picked up as it's costing the collectors a packet to fuel their vans.
It's very simple. STOP BUYING SO MUCH STUFF in the first place. Seriously. It needs to stop. Not just for charity shops, but fast fashion rubbish is killing the planet. And if you do decide to donate to charity, take a long look at what you are buying and consider whether you would pass it to your mum, sister, friend. If not, don't give it to the charity either!
Finally, charity shops are not just about clothes. The last place I volunteered took more on bric-a-brac, toys and games as we did on clothing.