get posts from custom taxonomy

The REST API is just an interface for the underlying WordPress functions. In this case, whatever you pass as the filter parameter is just passed through as a query argument to WP_Query. Since WP_Query does not support the same syntax for custom taxonomies as it does for categories and tags, it is not possible to filter by term ID with a top-level argument. This is unlikely to change unless WP Core changes. In fact querying by taxonomy slug is deprecated since WP v3.1 and you should use tax_query instead. However, tax_query is currently not a supported filter argument of WP REST API because it is considered slow. In order to use it you need to use the rest_query_vars filter like so

add_filter( 'rest_query_vars', function( $vars ){
	$vars[] = 'tax_query';
	return $vars;
} );

Your request URL would look like this:

http://punjabireel.com/wp-json/wp/v2/posts?filter%5Btax_query%5D%5B0%5D%5Btaxonomy%5D=poet&filter%5Btax_query%5D%5B0%5D%5Bfield%5D=id&filter%5Btax_query%5D%5B0%5D%5Bterms%5D=3