Hide restricted posts from queries

Hi,
Since UM version 2.2 their query strategy does not match our needs.
We cannot use the “404 page” when restricting a post. Instead we need to redirect the user to the login page when he/she receives a newsletter with that restricted post.

I think this can be easily solved using pre_get_posts in functions.php. But I unknow where in WP data tables are stored the restrictictions (roles) to the posts. I did not find any documentation about it and until now I did not find the arrays/elements by inspecting the WP database with phpMyAdmin.
Thanks for any hint