How to get the revisions list of a node and delete them all?
Yes, as you says there is a list of modules that can do this in Drupal 7 like:

Node Revision Delete


  The Node Revision Delete module lets you to track and prune old
  revisions of content types. Define the max amount of revisions to keep
  per content type. Run on Drush, cron run or on a different period
  (daily, weekly, etc).


Node Revision Restrict


  The Node Revision Restrict module allows you to restrict revisions of
  node for each content type. This module has an admin interface to
  specify node revision threshold for each content type available. The
  possible setting include node type with corresponding revision limit,
  provided on the Node Revision Restrict configuration form. Note that
  this module does not and cannot delete the current revisions of a
  node.


Node Revision Cleanup


  Node Revision Cleanup is designed for really big databases. And site
  administrators who are comfortable with Drush. The concept behind this
  module is to be able to run multiple invocations at the same time, via
  Drush to maximize the number of node revision deletions per second.


Revision Deletion


  This modules enables the ability to mass delete aging node revisions.
  Possible settings include node type, the age of node revision before
  being deleted, along with a Cron frequency setting. The revisions may
  be deleted after review on an administer page as well. This module
  will NEVER delete the current revision of a node, nor will it allow
  you to do so.


And for Drupal 8, well at this moment there is not module for do that, but you can use:

To get the revision list of a node:

$vids = \Drupal::entityManager()-&gt;getStorage('node')-&gt;revisionIds($node);


See Nodes now have a storage controller; node_revision_list() removed.

To delete a revision revision you can use the node_revision_delete() function, but at the end see the function code:

function node_revision_delete($revision_id) {
  \Drupal::entityTypeManager()-&gt;getStorage('node')-&gt;deleteRevision($revision_id);
}


So, is better if you just use:

\Drupal::entityTypeManager()-&gt;getStorage('node')-&gt;deleteRevision($revision_id);