Update: It looks like support for custom taxonomies filtering will come to version 6.0. Should also be shipped with Gutenberg 12.5 according to:
https://github.com/WordPress/Documentation-Issue-Tracker/issues/283
The below work-around should still work for other more complex types of custom filtering.
Looking into render_block_core_post_template() we can see it calls build_query_vars_from_query_block() (previously named construct_wp_query_args) to setup the query arguments of WP_Query from the Query` block properties.
From there I don't see it supporting custom taxonomies for the secondary
query ... yet!
Work-around-idea: For the Query Loop:
add a search keyword for using custom taxonomies, e.g. :query-motor-electric:
and write a plugin to handle this:
// Replace :query-motor-electric search keyword for a custom taxonomy query.
add_action( 'pre_get_posts', function( \WP_Query $q ) {
if ( $q->is_search() && ':query-motor-electric' === trim( $q->get( 's' ) ) ) {
// Custom taxonomy query.
$tax_query = array(
array(
'taxonomy' => 'motor',
'field' => 'slug',
'terms' => 'electric',
),
);
$q->set( 'tax_query', $tax_query );
// Clear search, unset search query variable or use a stop-word filter.
$q->set( 's', '' );
}
} );
or extend this further to support a dynamic keyword.
Example Block code for the Query Loop:
<!-- wp:query {"queryId":1,"query":{"perPage":3,"pages":1,"offset":0,"postType":"post","categoryIds":[],"tagIds":[],"order":"desc","orderBy":"date","author":"","search":":query-motor-electric","sticky":""}} -->
<div class="wp-block-query"><!-- wp:post-template -->
<!-- wp:post-title /-->
<!-- wp:post-date /-->
<!-- wp:post-excerpt /-->
<!-- /wp:post-template --></div>
<!-- /wp:query -->
where the relevant search part is:
<!-- wp:query {...,"query":{...,"search":":query-motor-electric"}} -->