~~~
My very crude observations so far is,
that a lot of those sub 1 sat/vB fee rate transactions are data spam, where it might not be important when it is confirmed. As mempools eventually dry out during a day or weekend but multiple times during a week these days, it makes a lot of sense to me to use sub 1 sat/vB fee rate.
I would do the same for transactions that aren't urgent. Why waste precious sats?
I believe not. If the mempool is congested by 0.10 sats/vB, then you're probably right. But current transactions with sub 1 sat/vB fees are actually using Bitcoin, although they're not in a hurry to have their transactions in the next block. They merely made their transactions and they're willing to wait for more than an hour to have them comfirmed.
Here's a block with some sub 1 sat/vB transactions,
https://mempool.space/block/00000000000000000000a74e77bb35e8691d7740249bba181c4bbdc96d9b2d74?page=41