Thing like that exist and are not new. TrueCrypt had such a function 15 years (or longer) ago already.
The trick is basically to have two partitions in the encrypted volume and depending which key you enter it opens one or the other. The filesystem needs to be given fake data about available free space to hide the “other” one.
I use the family plan with my wife. So we end up at $7 per month per user. Which IMO is ok. Given how important search is for our every day internet usage, I can accept this price.
I rarely have to jump to the second page of the search results to get what I want, so I am really happy with Kagi.
LLM is not knowledge.