Thanks for the in-depth response, Jennifer. This is very helpful.
I can echo this. I’d like to hide this functionality from my users to clean up the interface if possible.
This is a bit late, but I think there’s another way to solve this.
I think the most WordPress-y way to accomplish this is to add the following to your theme’s /functions.php:
add_post_type_support('forum', array('thumbnail'));
This will add the featured image metabox to all forums. See here for more details:
https://codex.wordpress.org/Function_Reference/add_post_type_support
