Have they banned it again?
See BoingBoing article from Oct 4th:
"Apple reverses ban on HKmap.live app tracking Hong Kong protests & police"
At first it wasn't exactly a ban, it was rejected before even going online [1]. Then they reversed the rejection.
[1] https://twitter.com/hkmaplive/status/1179108329240424448
Often a story gets held up while being written and doesn't account for developments in last few days. It is just confusing as they don't state that this would be Apple undoing a previous reversal in the fate of this app.
Update: ok, Apple did initial reinstated it and has removed it again.
https://techcrunch.com/2019/10/09/apple-pulls-hkmap-from-app...
If you want to build the source code and install the app to a device, you still need at least a Dev account ($99/yr) and macOS+Xcode. Without an account you won't have the provisioning and certificates to install on a "Dev" device. You are also limited to just 100 test devices I think under a regular dev account.
Another option is to crack the phone, then you can just go through third party repos to fetch the app.