My parents were solidly middle class.
Both went to university. Both travelled before I was born, living abroad for a bit.
Growing up we lived in a good area. My Dad had a senior management job at his company and was on a relatively good wage. My mum worked part time from when I was about 10. We didn't go on a foreign holiday until I was 11 and that was to visit my grandparents abroad so no accommodation to pay. We rarely got a takeaway never mind ate out - there wasn't many places to eat out locally. We had a spectrum when that came out. We had our first family computer in the early nineties and had broadband earlier than most. We had two second hand middle of the row family cars without fancy badges. They got screwed by Black Friday and Endowment mortgages so ended up working until retirement age but are now reasonably well off. And we were not badly off all things considered.
Compare that with expectations now of what middle class looks like.
Houses in the same area are astronomical. There are many more places to eat out - the shops that closed were the video store, the local hardware store, the greengrocers, the off licence, the sweetshop and several other product stores which now are covered by Amazon and the major supermarkets. A number of the pubs have gone under.
Now the idea of middle class is you go abroad several times a year, you eat out or get a takeaway regularly. You buy way more clothes (clothes are much cheaper now but this is made up for in the volume we buy). And it's way more brand intensive than even we were young. Your kids do way more clubs and they have childcare which wasn't available when I was growing up. You all have expensive devices - a laptop each and a TV in every room is standard. Never mind the subscription channels which we never had. Lots of the kids round here are tutored for various reasons (some for grammar entry, some for SATS, some to catch up, some for GCSEs). That was never a thing previously.
The expectation of what middle class is, is much much higher having peaked about 15 years ago and we are still trying to aspire to that lifestyle.
But the house prices continued to grow and the ability to maintain the lifestyle and the house has shrunk.
I don't think 'being middle class' has changed as such.
I do think there was an affluence bubble that ran out.
Lots of studies have shown this divide sits between age 44 and 48 (DH and I are on both ends of this). His school peers are all much worse off than my school peers. He's younger. If you are older than 48 you are on the right side of this. If you are younger than 44 you are on the wrong side of this. What's this about.
Well it's simple. The defining feature is whether you had purchased a property before 2008 or after. It's ALL about that economic crash. That's it. That's the only real common factor. That's why if you were working class but had a house before 2008 you could well be doing better economically than someone middle class who is younger.
But I don't think this means you have dropped class. I think it highlights what happened in the 2000s and early 2010s - there was a generation really living beyond it's means and not paying enough in infrastructure investment and we are paying the price for this. This was the era of PFI and kicking the can down the road and paying astronomical prices for the privilege of doing this. This was the era of low house building by the state. And then we wonder why the fuck we are in this mess now.
It's not rocket science. We have a census every ten years to monitor demographics and to plan from - we ignored these issues. We've barely had a large infrastructure investment in the last 25 years.