Slim SEO adds “noindex” to robots meta although it shouldn’t

There are some cases where the plugin auto add noindex robots tag. Please check if this is your case:

https://docs.wpslimseo.com/slim-seo/meta-robots-tag/

What is the type of your page? Is that a normal page or a custom post type? Does it have content (in the WP editor)? Do you use any page builder plugin to build it?

Does the term have any posts? If it doesn’t, e.g. no posts belong to a term of that taxonomy, then the plugin see that’s an empty archive page, and thus, will output the norobots tag.

Yes, please try this snippet:

add_filter( 'slim_seo_robots_index', function ($value) {
	if ( is_tax( 'your-taxonomy' ) ) {
		$value = true;
	}
	return $value;

} );