Google search suggests https://cloud.google.com/appengine/docs/standard/python/tools/downloading-source-code
However this is deprecated
Google search suggests https://cloud.google.com/appengine/docs/standard/python/tools/downloading-source-code
However this is deprecated
As you mention, this function is deprecated. However, there are some tricks that you can use in order to obtain the code of your application as it's uploaded to a bucket when you deploy.
I suggest you take a look as this other thread as this is explained into a bit more detail.
Hope you find this useful.