src/admin/class-papi-admin-ajax.php
public function get_terms() {
		$query    = papi_get_qs( 'query' ) ?: [];
		$query    = is_array( $query ) ? $query : [];
		$taxonomy = papi_get_qs( 'taxonomy' ) ?: '';

		$args = array_merge( $query, [
			'fields' => 'id=>name'
		] );

		$terms = [];
		if ( taxonomy_exists( $taxonomy ) ) {
			$terms = get_terms( $taxonomy, $args );
		}

		wp_send_json( $terms );
	}
public function get_terms() {
		$query    = papi_get_qs( 'query' ) ?: [];
		$query    = is_array( $query ) ? $query : [];
		$taxonomy = papi_get_qs( 'taxonomy' ) ?: '';

		$args = array_merge( $query, [
			'fields' => 'id=>name'
		] );

		$terms = [];
		if ( taxonomy_exists( $taxonomy ) ) {
			$terms = get_terms( $taxonomy, $args );
		}

		wp_send_json( $terms );
	}
Get terms via GET.

GET /papi-ajax/?action=get_terms