Remove Featured Images from Posts Older Than a Certain Date

I think you might be jumping the gun with actively discarding the data here. What if next month trolls go away, you change your mind, and want all those featured images back?

I would just suppress them on front end by editing template and making thumbnail output conditional or filtering API with something like this:

if ( ! is_admin() ) {

    add_filter( 'get_post_metadata', function ( $null, $post_id, $meta_key ) {

        if ( '_thumbnail_id' === $meta_key ) { // add check for date or whatever
            return false;
        }

        return $null;
    }, 10, 3 );
}

This way you retain all the existing data, just prevent featured images from showing. If you change your mind just remove the check and everything is back.