Drupal 9 plan

Using the drupal-check tool, I don't actually see any deprecated code in this module.

While the D8.7 cycle is still active, you probably just want this:

--- a/language_hierarchy.info.yml
+++ b/language_hierarchy.info.yml
@@ -3,5 +3,6 @@ type: module
 description: 'Allows users to configure inheritance between languages.'
 package: Multilingual
 core: 8.x
+core_version_requirement: ^8 || ^9
 dependencies:
   - drupal:language

If you want to wait until the D8.7 cycle is done (which I believe is in June), you could do this:

--- a/language_hierarchy.info.yml
+++ b/language_hierarchy.info.yml
@@ -2,6 +2,6 @@ name: Language Hierarchy
 type: module
 description: 'Allows users to configure inheritance between languages.'
 package: Multilingual
-core: 8.x
+core_version_requirement: ^8.8 || ^9
 dependencies:
   - drupal:language

However, the 8.7 cycle ends at the same time D9 is launched, so I'd say the first version is probably preferable for now. That's what I've done in some of my other modules.

I've attached a patch for both for you to weigh the options.

As it doesn't appear there is a composer.json file for this module, I believe these are the only changes necessary at this point.

It seems weird that the build is saying it failed testing, considering there are no tests in the module. Placing back to in review for re-assessment here.