REST resources now have a single "GET" route for all supported formats

REST resources no longer have one route per supported format for the GET method.

For example here are the routes for the Node entity resource:

Drupal <8.4.0 (8.0.x, 8.1.x, 8.2.x, 8.3.x)

1. rest.entity.node.GET.hal_json
2. rest.entity.node.GET.json
3. rest.entity.node.GET.…

Drupal >=8.5.0

rest.entity.node.GET

So you can now generate the URL to access node 5 in json format using:

$url = Url::fromRoute('rest.entity.node.GET')
  ->setRouteParameter('node', 5)
  ->setRouteParameter('_format', 'json');

Backwards compatibility for clients (HTTP API): ✔️

Since the previous per-format routes all had the same system path this change should not affect any clients using the REST resources.

Backwards compatibility for Drupal code: ✔️

Not even Drupal code that relies on the (now deprecated) route names needs to be updated — this continues to work just fine:

// Deprecated, but still works until Drupal 9!
$url = Url::fromRoute('rest.entity.node.GET.json')
  ->setRouteParameter('node', 5);

(Backwards compatibility is provided through an outbound route processor.)