The WP_Filters API has a huge downside. It doesn't support unhooking of namespaced functions/class functions and so on to the best of my knowledge, but I refuse to believe that nor the core team, nor someone else came up with a way to do it.

I've read, searched but to no avail, the only things that are out there don't actually work.

An example of what I'm trying to achieve:

Class Test
{
    public function __construct()
    {
        add_action( 'init', [$this, 'testFunction'] );
    }

    public function testFunction()
    {
        echo 'Hey';
    }
}

new Test;

remove_action( 'init', ['Test', 'testFunction'] ) // and so on.

So, when we're actually hooking that function, what WP does internally is add_filter creates a unique hash for an object, in order to identify it by using _wp_filter_build_unique_id/, the problem with this function or rather PHP, is that it uses spl_object_hash to build that identity. This function doesn't give you an unique hash through requests, so, if you do remove_action( 'init', 'computedHashForTheObjectAndFunction' ), it will not work, because on the next request, it's gonna be a new one.

My assumption is that you can remove a function if you have that object's instance, but that's a rare, rare sight.

What can I do?

Edit: Yup, if you have access to the instance of the class in the same request, you can basically re-hash it to the same value and it will be able to remove the hook for you, so, inside my class, if I do remove_action( 'init', [$this, 'testFunction'], 10 );, it will work. Outside of that, though, is as I've said - impossible without an instance. Re-intializing the object just to get its identity is very, very bad because you might end up with unwanted functionality, so, that's out of the question.