If you are going to make an unauthorized client for a service it’s probably better to make it open source. That way other developers are able to take it over if the original developer loses interest.
That is exactly what happened to PoGo, the 3rd party Pokemon Go client for Windows 10 phones and PCs, created by ST-Apps.
After some issues with the Hashing algorithm that developer stopped updating the app, but another, mtaheij, has now taken over, and a new version of the app has recently been released.
That version relies on buying a hashing key from another 3rd party service (somewhat similar to the 3rd party Snapchat client) but otherwise seems to be rather functional, except for the pretty 3D and AR effects.
The latest version, v1.2.10 has just been released with the ability to do the tutorial in the app, picking a team when visiting a Gym for the first time and even deploying Pokemon to Gyms if there are empty spaces there.
Again, like the Snapchat app, there is no guarantee the app will not get you banned from the Pokemon service.