TemplateDefinition has a property visible that hides the template in Admincentral. The default template availability strategy checks this flag and excludes templates that have it set.

In DefaultTemplateAvailability:

        // Templates with no visible property, or visible=false are not available.
        if (templateDefinition.getVisible() != null && !templateDefinition.getVisible()) {
            return false;
        }

This is probably not an issue since users can just not list the template in the site definition templates availability settings. But for consistency non-visible templates should be hidden even if listed in the site config.