I think "lots" is pretty load-bearing here. How many younger people, precisely, have moved back home? If half of them have, I agree that's a big paradigm shift; if 5% have, I'm not sure that adds up to something very far outside normal variability.
https://www.pewresearch.org/fact-tank/2020/09/04/a-majority-...
https://www.pewresearch.org/fact-tank/2020/09/04/a-majority-...
Of course, it's likely that this impact isn't going to be uniform, so that doesn't mean individual neighborhoods (or even individual cities) have no cause for concern.
It appears like quite a few. I can see into a lot of different apartments from where I live in SF and many, many more are empty. This might also be due to AirBnBs.
The real estate rental prices have fallen, but they're still pretty sticky - I predict they are going to fall more.