It means that you have to click the power button in a call to switch off the screen.
And, each time you get a message or change the volume, check the screen is off again and if not, power it off again… (that last part is annoying, the first one can be quickly learned)
It is not, but I could include it, I have now some experience with that. There are a couple of problems with retro-fitting: One is that I wouldn’t enable it by default so I don’t surprise existing users with an option that would potentially make them lose their existing home access. So you’d have to enable it after the Tutorial/first boot manually (that’s what I did for Zenfone 8 too).
The other “problem” is that you need to pre-allocate the max size of the home partition. This might be OK for Mi Note 10, maybe there aren’t multiple versions of the hardware (with different storage sizes) but it is a problem contributing to the fact that I haven’t enabled it by default on Zenfone 8.
This latter problem is probably not existing with Jolla encryption, but that requires a flashable lvm volume as the output of the image build, and the knowledge how to do that is not yet acquired by me (and is again not very upgrade friendly…)