Hi Erich,
Though it is possible to filter nav menu and add Item, I will prefer another approach.

I will create a page call it say “my-subscriptions” and then on “template_redirect” action check if the user is logged and is trying to visit this page redirect to the link you wanted to link earlier.

Something like this

add_action( 'template_redirect', 'buddydev_bbp_redirect_tosubscription_page' );

function buddydev_bbp_redirect_tosubscription_page() {
	
	if( !  is_user_logged_in() || ! is_page( 'my-subscription') ) {
		return ;
	}
	
	$url = bbp_get_user_profile_url( get_current_user_id() ) . 'forums/subscriptions/#bbp-author-unread-topics';
	
	wp_safe_redirect( $url );
	exit(0);
	
}

Hope that helps.