It does’t work because you can’t use set to change all query vars. Simplest way to do the trick is set 's' to an empty string:
add_action( 'pre_get_posts', function( $query ) {
if ( ! is_admin() && $query->is_main_query() && $query->is_search() ) {
$taxquery = array( ... );
$query->set( 'tax_query', $taxquery );
$query->set('s', '' );
}
});
In this way, WordPress will still consider the request a search, so the search.php template will be used to show results and if you call is_search() the result will be TRUE.
If you want that WordPress will not consider anymore the request a search, you need to also manually unset $query->query_vars['s'] and set $query->is_search to false:
add_action( 'pre_get_posts', function( $query ) {
if ( ! is_admin() && $query->is_main_query() && $query->is_search() ) {
$taxquery = array( ... );
$query->set( 'tax_query', $taxquery );
$query->set('s', '' );
unset( $query->query['s'] );
$query->is_search = FALSE;
}
});
Doing that, WordPress will not anymore consider the request a search, and will use index.php as template.
If you want to use another template, e.g. taxonomy.php you need to use 'template_include' filter:
add_action( 'pre_get_posts', function( $query ) {
if ( ! is_admin() && $query->is_main_query() && $query->is_search() ) {
$taxquery = array( ... );
$query->set( 'tax_query', $taxquery );
$query->set('s', '' );
unset( $query->query['s'] );
$query->is_search = FALSE;
add_filter( 'template_include', function() {
return locate_template( 'taxonomy.php' )
}, 0 );
}
});