Has the Discourse invite feature been switched off ?
To answer my own question, no it hasn’t.
It has been obscured for some reason (in the same manner as the “+ New Topic” button,) and moved to the topic [ 
Share ] popup menu. (The button all the way at the bottom of the topic thread. Not on each post’s share popup menu.)
(Note: I use DarkReader extension for Chrome if you wonder why the menu image is so dark.)