bbp_get_topic_tag_names( int $topic_id, string $sep = ', ' )

function bbp_get_topic_tag_names( $topic_id = 0, $sep = ', ' ) {
	$topic_id   = bbp_get_topic_id( $topic_id );
	$topic_tags = array_filter( (array) get_the_terms( $topic_id, bbp_get_topic_tag_tax_id() ) );
	$terms      = array();
	foreach ( $topic_tags as $term ) {
		$terms[] = $term->name;
	}
	$terms = !empty( $terms ) ? implode( $sep, $terms ) : '';

	return apply_filters( 'bbp_get_topic_tags', $terms, $topic_id );
}