Drupal 9 Compatibility.

README.txt

@@ -12,5 +12,5 @@ This module has absolutely no functionality; it simply disables the warning.
 
 To install:
 - Place the module in your modules/contrib directory.
-- Enable it from Admin -> Extend (admin/modules).
+- You can only enable this by updating core_extensions.yml or enabling it via Drush. It is hidden from the UI.
 

composer.json

+{
+    "name": "drupal/update_notifications_disable",
+    "type": "drupal-module",
+    "license": "GPL-2.0-or-later",
+    "require": {
+        "drupal/core": "^8 || ^9"
+    },
+    "extra": {
+        "branch-alias": {
+            "dev-8.x-1.x": "1.x-dev"
+        }
+    }
+}

update_notifications_disable.info

-name = Update Notification Disable
-description = "Turns off core's update notification and removes warning about the Update status module not being enabled."
-core = 8.x

update_notifications_disable.info.yml

+name: 'Update Notification Disable'
+type: module
+description: "Turns off core's update notification and removes warning about the Update status module not being enabled."
+core: 8.x
+core_version_requirement: ^8 || ^9
+package: Other
+hidden: true