Why is it not possible to install Windows 7 on any Mac models released since 2015?
Question: Could somebody confirm why this is the case - what was removed as part of these Macs that Windows 7 previously relied upon for a successful installation?

Answer: On Macs where Apple officially supported Windows 7, the Boot Camp Assistant would set up Windows 7 to BIOS boot. Starting with the 2015 model Macs, Apple starting eliminating the BIOS from the firmware. So, there may still be a 2015 or newer Mac with a BIOS out there somewhere, but officially BIOS booting ended starting in 2015. Apple starting using the EFI standard to boot macOS before there was a UEFI standard. Apple never completely implemented the EFI standard. Evidently, some part of the standard that Windows 7 needed did not exist in firmware on Mac computers. AFAIK, Mac computers could only BIOS boot Windows 7.