It seems like there's no way to query for objects that simply have a relationship ID.
It would be great if the 'relationship' query argument could accept EXISTS or NOT EXISTS. Similar to meta_query. Instead of passing it via "to" or "from" maybe introduce the "compare" param?
So for a modified example based on the documentation, something like this:
$query = new WP_Query( [
'relationship' => [
[
'id' => 'events_to_bands',
'compare' => "EXISTS"
],
],
] );
Or if there is some current way to get such posts, please let me know. Thanks!