I recently migrated from Windows 8 / VS2013 to Windows 10 / VS2015. After doing so I can no longer build Windows 8 store apps - I'm forced to upgrade to Windows 8.1 if I want to build the project.
However, I maintain two builds of my app - one for Windows 8 and one for 8.1. I have a number of customers who refuse to update to 8.1 (No, I have no idea why) and I don't want to leave them with an outdated version. I've already upgraded the project to 8.1 and keep that separate.
Is there any way to build a Windows 8.0 app in Visual Studio 2015, without the forced upgrade?
Visual Studio 2015 does not support building Windows 8 Store apps. As you indicated it will only migrate them to 8.1. You must use VS2013 to maintain Windows 8 Store apps and you can only use VS 2012 to create any new ones.
This is covered in the Visual Studio 2015 compatibility support page.
https://www.visualstudio.com/en-us/products/visual-studio-2015-compatibility-vs.aspx