I've mulled/wished for this for years. Also useful at borders, where in the past I have actually been asked (required) to unlock phones and laptops. Generally you have no rights whatsoever there.

Those shadow accounts would need to be 'lived in' to pass those border checks. My worst experience was traveling with new, obviously burner devices

border agents were extremely suspicious.

I forgot what it was called, but someone create an encrypted file system where you could never be certain all files were decrypted. You could enter one password and files A B and C would be revealed and accessible, then you could enter another password and files D E and F would be revealed, and again, another password would reveal file G, etc.

The file system was just a big blob of seemingly random bytes, but when processed with the right password, certain patterns would be revealed, those patterns being the files. This brought with it the possibility that files would be lost, because when writing files with password 1, files encrypted with password 2 might be overwritten. Several copies of each file were stored to protect against this, but you could still lose files.

There are some philosophical / legal issues with such a file system, because you can never prove that you've decrypted all the files. If prosecutors wanted to claim that you had more files on the filesystem, there's no way you could disprove it, because you can never prove that you've decrypted everything. Hopefully people would be considered innocent until proven guilty, but believing the law always works that way is naive.

EDIT: It's called deniable encryption: https://en.wikipedia.org/wiki/Deniable_encryption

I used to have an android launcher years back that did just that thing, actually. It ran different instances of the home page based on what password you entered. You could access other instances when logged in via a 3-finger side drag, but it was able to be disabled. I don't recall what it was called anymore but I had to have been using it back when I had a Galaxy S8 or even older.

I had something similar on my laptop with encrypted volumes and duress passwords. So my documents folders were all on an encrypted volume and opened by the standard super hard password. The duress password was much easier and contained a skeleton structure to look legit. The idea was that if anyone brute forced the password it would just find the duress folder first and hopefully no one would look further. Seems like overkill but I was traveling to China for business so necessary. I did however use a burner phone as opposed to my real cell.

No, but there is at least one app out there that lets you set a panic code that will wipe the phone when used

I doubt that's how the password is used for. More like they copy all contents of the phone and ask the password to go through encryption. The data is already there, accounts don't matter.

This is also the reason why it's no good to have a dead man's switch or the like, as in a certain password just wipes everything. You'd just get arrested for destroying evidence and they continue from a copy.