Enable post revisions for a specific post

Question

Since the check is made in wp_revisions_to_keep($post) in wp-includes/revision.php, and the result is run through a filter, you should be able to do something like this:

<?php
function wpse_289553($num, $post) {
        if(in_array($post->ID, array(123, 456, 789))) {
            return -1; 
        }
        return $num;
}
add_filter("wp_revisions_to_keep", "wpse_289553", 10, 2);

Set your post IDs in that array and adjust the returned number to how many revisions you’d like to keep. -1 means “all of them”.