The new desktop client does not have "Keep me signed in" as an option because that is its default and it can't be altered.

This is not documented for the new desktop client which can lead to people wasting time trying to sort out what is happening.

What does this do:
If you don't sign out but simply close the app window next time the protonmail app starts you are not challenged for a password. This still occurs if you reboot the laptop/pc/virtual machine.

The only time you have enter a password is when you reset the process by signing out from the app itself.

This may be fine for the home environment but not for business. For security reasons best to have logons for each desktop application and not rely upon locking your desktop all the time.

For business and some home users it be best to make it optional.

Reference: Service Desk request 2751628)