I noticed this on YT as well, and tested this logged out to determine if this was a more tailored algorithm for me, or if there really was a trend advocating for these style of phones. It did appear for a time (around the summer of 2023) that an increase of this type of content were making the rounds.
It's possible that Nokia with it's limited marketing budget had not capitalized on the trend, and simply missed out thus the large decline.
For what it's worth, I every day carry a Nokia 6300, and have an iPhone 13 Mini as a backup.