For those who remember their Microsoft account credentials, there is a technical backdoor. Microsoft still hosts the files on Content Delivery Networks (CDNs), but the storefront is gone. You can use a PC tool to query Microsoft’s servers directly.

Get-AppxPackage | Select Name, PackageFamilyName

Once you have your .XAP or .APPX file and the SDK tools ready: How to install appxbundle on windows phone - GitHub Gist

"The app downloads but won't install on my Lumia 950." Solution: 64-bit ARM compatibility issues. WP8 XAP files will NOT run on Windows 10 Mobile ARM64. You need the correct architecture: ARM (for old phones) vs ARM64 (for Lumia 950/950XL).