When I upgraded to PHP 7.2 rather than 7.3, my website didn’t break. Suppose I should find out if WordPress supports this version !
Using cPanel File Manager I can see that /wp-includes/kses.php exists.
I deleted all plugins except MetaSlider which is deactivated. (ours is a very simple website).
My theme is twentynineteen.
Upgrade to PHP 7.3 still breaks website with same error.
