Adding a flag to Menu Content Link to display as a non-link
The module Special menu items allows you this, even some control over the HTML output. 

This is now in Drupal core (although Docs need some updating): 


  This features is in core for >=8.2.0. Create menu without link by entering route: &lt;nolink&gt;.


That is, you enter into Link: 

route:&lt;nolink&gt;

and select: 

Show as expanded

if you need it for drop-downs.