How to make menu items of restricted pages visible to all user roles?
Ah look, Always Visible already had an issue regarding D8: Drupal 8 port of Always Visible. Some nice person created a Gist you can easily build a custom module from to always display all menu items.

I've done that for you and put it on GitLab: Always Visible (D8). Simply download the Zip file, unpack it, rename the folder to always_visible and put it into your /modules/custom folder, enable the module, done.