src/ResourceType/ResourceType.php           | 16 ++++++++--------
 src/ResourceType/ResourceTypeRepository.php |  8 ++++----
 src/Routing/Routes.php                      | 15 +++++++--------
 3 files changed, 19 insertions(+), 20 deletions(-)

diff --git a/src/ResourceType/ResourceType.php b/src/ResourceType/ResourceType.php
index 355028a..1ea2a00 100644
--- a/src/ResourceType/ResourceType.php
+++ b/src/ResourceType/ResourceType.php
@@ -61,7 +61,7 @@ class ResourceType {
    *
    * @var bool
    */
-  protected $isValidatable;
+  protected $isMutable;
 
   /**
    * Gets the entity type ID.
@@ -203,16 +203,16 @@ class ResourceType {
   }
 
   /**
-   * Whether resources of this resource type are validatable.
+   * Whether resources of this resource type are mutable.
    *
-   * A resource type may not be modified (created, updated or deleted; POST,
-   * PATCH or DELETE) when it is not validatable.
+   * Indicates that resources of this type may not be created, updated or
+   * deleted (POST, PATCH or DELETE, respectively).
    *
    * @return bool
-   *   TRUE if the resource type's resources are validatable. FALSE otherwise.
+   *   TRUE if the resource type's resources are mutable. FALSE otherwise.
    */
-  public function isValidatable() {
-    return $this->isValidatable;
+  public function isMutable() {
+    return $this->isMutable;
   }
 
   /**
@@ -237,7 +237,7 @@ class ResourceType {
     $this->deserializationTargetClass = $deserialization_target_class;
     $this->internal = $internal;
     $this->isLocatable = $is_locatable;
-    $this->isValidatable = $is_validatable;
+    $this->isMutable = $is_validatable;
 
     $this->typeName = sprintf('%s--%s', $this->entityTypeId, $this->bundle);
   }
diff --git a/src/ResourceType/ResourceTypeRepository.php b/src/ResourceType/ResourceTypeRepository.php
index e61ab60..88d0ad0 100644
--- a/src/ResourceType/ResourceTypeRepository.php
+++ b/src/ResourceType/ResourceTypeRepository.php
@@ -106,7 +106,7 @@ class ResourceTypeRepository implements ResourceTypeRepositoryInterface {
             $entity_type->getClass(),
             static::shouldBeInternalResourceType($entity_type),
             static::isLocatableResourceType($entity_type),
-            static::isValidatableResourceType($entity_type)
+            static::isMutableResourceType($entity_type)
           );
         }, array_keys($this->entityTypeBundleInfo->getBundleInfo($entity_type_id))));
       }
@@ -165,15 +165,15 @@ class ResourceTypeRepository implements ResourceTypeRepositoryInterface {
   }
 
   /**
-   * Whether an entity type is a validatable resource type.
+   * Whether an entity type is a mutable resource type.
    *
    * @param \Drupal\Core\Entity\EntityTypeInterface $entity_type
    *   The entity type to assess.
    *
    * @return bool
-   *   TRUE if the entity type is validatable, FALSE otherwise.
+   *   TRUE if the entity type is mutable, FALSE otherwise.
    */
-  protected static function isValidatableResourceType(EntityTypeInterface $entity_type) {
+  protected static function isMutableResourceType(EntityTypeInterface $entity_type) {
     return !$entity_type instanceof ConfigEntityTypeInterface;
   }
 
diff --git a/src/Routing/Routes.php b/src/Routing/Routes.php
index a4cfba0..34cef7b 100644
--- a/src/Routing/Routes.php
+++ b/src/Routing/Routes.php
@@ -140,7 +140,11 @@ class Routes implements ContainerInjectionInterface {
 
     // Collection route like `/jsonapi/node/article`.
     $collection_route = new Route('/' . $resource_type->getPath());
-    $collection_route->setMethods($resource_type->isLocatable() ? ['GET', 'POST'] : ['POST']);
+    $methods = $resource_type->isLocatable() ? ['GET', 'POST'] : ['POST'];
+    if (!$resource_type->isMutable()) {
+      $methods = array_intersect($methods, ['GET']);
+    }
+    $collection_route->setMethods($methods);
     $collection_route->addDefaults(['serialization_class' => JsonApiDocumentTopLevel::class]);
     $collection_route->setRequirement('_csrf_request_header_token', 'TRUE');
     $routes->add(static::getRouteName($resource_type, 'collection'), $collection_route);
@@ -197,7 +201,7 @@ class Routes implements ContainerInjectionInterface {
 
     // Individual read, update and remove.
     $individual_route = new Route("/{$path}/{{$entity_type_id}}");
-    $individual_route->setMethods(['GET', 'PATCH', 'DELETE']);
+    $individual_route->setMethods($resource_type->isMutable() ? ['GET', 'PATCH', 'DELETE'] : ['GET']);
     $individual_route->addDefaults(['serialization_class' => JsonApiDocumentTopLevel::class]);
     $individual_route->setRequirement('_csrf_request_header_token', 'TRUE');
     $routes->add(static::getRouteName($resource_type, 'individual'), $individual_route);
@@ -212,7 +216,7 @@ class Routes implements ContainerInjectionInterface {
     // Read, update, add, or remove an individual resources relationships to
     // other resources.
     $relationship_route = new Route("/{$path}/{{$entity_type_id}}/relationships/{related}");
-    $relationship_route->setMethods(['GET', 'POST', 'PATCH', 'DELETE']);
+    $relationship_route->setMethods($resource_type->isMutable() ? ['GET', 'POST', 'PATCH', 'DELETE'] : ['GET']);
     // @todo: remove the _on_relationship default in https://www.drupal.org/project/jsonapi/issues/2953346.
     $relationship_route->addDefaults(['_on_relationship' => TRUE]);
     $relationship_route->addDefaults(['serialization_class' => EntityReferenceFieldItemList::class]);
@@ -222,11 +226,6 @@ class Routes implements ContainerInjectionInterface {
     // Add entity parameter conversion to every route.
     $routes->addOptions(['parameters' => [$entity_type_id => ['type' => 'entity:' . $entity_type_id]]]);
 
-    // If a resource type is not validatable, make all its routes read-only.
-    if (!$resource_type->isValidatable()) {
-      $routes->setMethods(['GET']);
-    }
-
     return $routes;
   }