Hello everyone,
Though some of the content in that repo is a bit out of date at the moment, it can still be an asset for people who are looking at add custom map layers to their Arches installation, and hopefully we can get it updated soon enough.
If you have examples of map layers that you can share, please feel free to make a pull request on the new repo, or open an issue on it if there is anything related to custom map layers that you are having trouble with.
Adam