In drupal 7 you could use some contrib modules with a <nolink> tag to make subtitles in our menu. In drupal 8 I try to avoid contrib modules on my menu items. Here's a hint on how to get it to work.
Edit your menu item and add the following as the path:
route:<nolink>
Done! You now have a valid menu item without a link.
Saved you some valuable time?
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