It's the official spine-unity runtimes acquired from the offical spine-runtimes GitHub repo with no changes at all, except it's already bundled with spine-csharp runtime so it can be installed in Unity without throwing any dependencies error.
If you try to install spine-unity runtime via Package Manager:
https://github.com/EsotericSoftware/spine-runtimes.git?path=/spine-unity/Assets/Spine#[BRANCH-NAME/VERSION]
it will throw an errors saying you don't have spine-csharp present. You then proceed to install spine-csharp via Package Manager, but it couldn't because spine-csharp didn't have package.json that's required for Unity to identify a package.
The solution is, you need to bundle spine-unity and spine-csharp together so it can be installed via Package Manager.
Important
Change the [BRANCH-NAME/VERSION] to the version that you want.
You cannot have multiple Spine runtime at the same time because that is the original design intended.
https://github.com/kiraio-moe/spine-unity.git#[BRANCH-NAME/VERSION]