https://www.tenforums.com/tutorials/105104-add-remove-sign-out-option-windows-10-a.html
In Local Group Policy Editor (gpedit.msc), enable 'User Configuration > Administrative Templates > Start Menu and Taskbar > Remove Logoff on the Start Menu'
This prevents signing out the current user, but still allows switching to another user, and then a password is still required to switch back. Hmmm.