Khan Academy replied to my problem report that the subject area icons are now located on the Search page (Search button in the Khan-specific button row on the bottom of screen). I verified this in both apps. There is no explicit exit from this Search page, but pressing the Home button in the Khan-specific button row on the bottom of the screen gets you back there. My Android Samsung S10 phone running Android 10 also has a back button on a row of butttons below the Khan-specific buttons, and this one also works as an exit from the Search screen.
I don't like this change and would rather see the subject area buttons on the Home screen. I will be responding to Khan about this later this week.