What is the correct REST URL in Drupal 8?
The comment above is right - the answer for this is very much in flux, and this is a temporary hack - but nevertheless, if you need a REST endpoint today, you can do this:

Copy the entire extern directories into your web/libraries/civicrm directory.
Copy the template for civicrm.config.php into web/libraries.

Here are the commands I used:
cp -r &lt;drupalroot&gt;/vendor/civicrm/civicrm-core/extern &lt;drupalroot&gt;/web/libraries/civicrm/extern
cp &lt;drupalroot&gt;/web/modules/contrib/civicrm/civicrm.config.php.drupal &lt;drupalroot&gt;/web/libraries/civicrm/civicrm.config.php