WP_Query for a taxonomy value OR a custom post type meta field

What you’re trying to do — using the OR relation with the tax_query and meta_query parameters, i.e. to have a <tax queries> OR <meta queries> instead of <tax queries> AND <meta queries> command in SQL, unfortunately (without custom and possibly complex queries) is not possible in WP_Query for now, therefore that 'relation' => 'OR' in your query args (the $args array) is not going to work or even do anything.

However, what you can (and I would) do, is: Turn that metadata into a custom taxonomy which is better suited for grouping a collection of posts together just like in your case where you used the metadata to group posts that are “playlist-available” (?), and moreover, taxonomy terms can also be as simple as 1 and 0, and not necessarily need to be titles like Movies and Web Design. You could even create a custom metabox with a “Yes-or-No” checkbox which then updates the taxonomy terms for the post. (I mean, if you don’t like the default taxonomy UI/selector..)

So for example, register a taxonomy named playlist_available, and then add these terms to that taxonomy: 1 (“Playlist available”) and 0 (“Playlist not available”) — although I think you may not need the latter..?

And then for all existing posts that had the metadata _csl_playlist_available_to_all set to 1, remove that metadata from the post and then assign the post to the 1 term in the playlist_available taxonomy.

Then when you make your WP_Query, there’s no need for the meta_query and just add another clause to the tax_query parameter/array:

$args = array(
    'post_type' => 'csl_playlist_manager',
    'tax_query' => array(
        'relation' => 'OR',
        array( // selects posts that are in this taxonomy
            'taxonomy' => 'csl_source',
            'field'    => 'term_id',
            'terms'    => $strTax,
        ),
        array( // OR the ones with the playlist available
            'taxonomy' => 'playlist_available',
            'field'    => 'slug',
            'terms'    => '1', // or whatever the slug is
        ),
    ),
    // ... your other args.
);

And now that should give you the results you expected, and the query would also run faster than with the meta_query which could end up with various JOIN clauses — on small sites, you may not notice an obvious performance boost; but you would when your site gets larger (many posts, options, plugin data, traffic/users, etc.). ^{But a bit of disclaimer, I’m far from being a performance expert. 🙂}