I noticed that a plugin I am using has this hook during its rendering of a meta box in one of its settings tabs:
do_action( 'learndash_metabox_options_div_after', $metabox->settings_metabox_key );
I spent hours trying to figure out how to extend its meta box and display a couple additional fields, or add a meta box to this same tab, but finally gave up when I noticed this hook. So from this hook I just render my additional fields using the callback and save function that I wrote as I was trying to use add_meta_box, and all seems to work.
But can't help but think this is not good, and especially with the restAPI it can lead to trouble (since I didn't register these fields or anything). But why would there be a hook of this sort in the first place if it wasn't supposed to enable me to insert something into this meta box?