POSTing to EntityResource can now happen at /node, /taxonomy/term … instead of /entity/node, /entity/taxonomy_term …

When POSTing an entity using REST, you have to POST to /entity/{entity_type} (for example /entity/node or /entity/taxonomy_term) in Drupal 8.0.x, 8.1.x, 8.2.x and 8.3.x. This was forgotten in https://www.drupal.org/node/2199185.

Of course, you'd expect to be able to post to /node or /taxonomy_term. This is now supported in Drupal 8.4.x, for all entity types that have a create link template (for the https://www.drupal.org/link-relations/create link relation type) in their entity type annotation.

Old code that makes requests to /entity/{entity_type} continues to work, because a BC layer is in place: so POST /entity/taxonomy_term requests are automatically mapped to POST /taxonomy/term.

Note: only for the User entity type does this not yet work, due to other bugs blocking it. This will be fixed by #2851984: Add "create" link template to User entity type annotation, to allow POSTing to /user instead of /entity/user. So, for users, continue to use POST /entity/user, for now.

How does this work on the Drupal/PHP/server side?

In other words: just like the REST module in Drupal 8.0.x, 8.1.x, 8.2.x and 8.3.x has already been using the canonical link template of an entity type if it is specified in favor of its default (/entity/{entity_type}/{entity}), starting in Drupal 8.4.x, it will do exactly the same for the create link template (for the https://www.drupal.org/link-relations/create link relation type): it will use that instead of the default (/entity/{entity_type}).

Contributed modules that provide additional entity types

Contributed modules that provide entity types that also wish to take advantage of this merely need to specify this link template in their annotation, and it will start working, and they'll get a BC layer automatically too!
(We simply added this same link template to all of core's entity types.)