> [for instance, we have a field "sidebar" for nodes. of course, that
> field's content should be rendered outside of the normal "content", in
> a different region.][1] - Alex 18 hours ago

Instead of using the theme's layout and side bar region, you could use the one of the following modules and create a panel or display suite layout with a side bar region. Then you can insert the node fields and other blocks you need on the sidebar and on other regions that you create. 

### [Panels][3] 

> The Panels module allows a site administrator to create customized
> layouts for multiple uses. At its core it is a drag and drop content
> manager that lets you visually design a layout and place content
> within that layout. Integration with other systems allows you to
> create nodes that use this, landing pages that use this, and even
> override system pages such as taxonomy and the node page so that you
> can customize the layout of your site with very fine grained
> permissions.

### [Display Suite][2]

> Display Suite allows you to take full control over how your content is
> displayed using a drag and drop interface. Arrange your nodes, views,
> comments, user data etc. the way you want without having to work your
> way through dozens of template files.


 [1]: https://drupal.stackexchange.com/questions/199384/whats-best-practice-to-show-a-nodes-fields-in-different-regions-in-drupal-8#comment-241122

 [2]: https://www.drupal.org/project/ds
 [3]: https://www.drupal.org/project/panels