If you mean how to ignore <!--nextpage--> in the feed content, then we can adjust this approach for feeds:
/**
* Disable content pagination for feeds
*/
add_filter( 'content_pagination', function( $pages )
{
if ( is_feed() )
$pages = [ join( '', (array) $pages ) ];
return $pages;
} );
where $pages is an array that contains the content parts.