Adding the method as PHP code will be done in Symfony 5. But you can already find its signature in the docblock:

So for now old method removed instead of to be marked as deprecated.
And no any new methods available until 5.0

I've got the next error in PHPUnit tests.
"Trying to configure method "getReachableRoles" which cannot be configured because it does not exist, has not been specified, is final, or is static"

And, as I understand, there is no other methods that could be used and exists in this interface.
And this interface is useless until version 5.0

Why this method has been already removed?
Comment in implementation said that it should be removed in version 5.0, but not in 4.3.

#30388 (comment) contains some more information

Removing a method from an interface is a BC break. It causes exceptions such as:

Prophecy\Exception\Doubler\MethodNotFoundException: Method Double\RoleHierarchyInterface\P19::getReachableRoles() is not defined.

That's only an issue for mocks, not for real code.
You might need to create a TestRoleHierarchyInterface that extends the native one with the added method.
I did something similar in https://github.com/symfony/symfony/pull/31597/files#diff-13b6c6ea7cdaba73a859c38af4231fd6R70 (for another reason)
That could as well be considered an issue with mocking libraries. It would actually be great if they could handle @method annotations or similar.

No bug here to me.

For what it worth, I stop mocking RoleHierarchyInterface and start using RoleHierarchy without mock. Simpler!
https://github.com/symfony/symfony/pull/31824/files#diff-953a0eb8ac77c56ed64c155b3c5789feR45

Mocking is just one of the ways that the problem would manifest itself.

Consider this, it was a real method defined on the interface before, as far as PHP is concerned. In other words, causing the return value of method_exists to be changed is a BC break.

Adding a new method that only exists in PHPDoc @method is fine. Removing an existing actually defined method is not.

According to https://symfony.com/doc/current/contributing/code/bc.html#changing-interfaces @teohhanhui is right.

I think we can close this as it's solved in 5.0 and people shouldn't need to mock the interface as explained in #31814 (comment)