Apple has confirmed that the next major release of macOS, Big Sur, will identify itself as both version 10.16 and 11.0 according to context. This should put the minds of many at rest, and ease any pressure on updating apps and scripts.
For apps built with Xcode, the version returned depends on which version of its SDK they were built with. SDK 10.15 and earlier will consistently respond that Big Sur is major version 10 and minor version 16. This ensures that all existing apps should see Big Sur as simply an incremented minor version, as we had expected before WWDC this year.
Build an app with a new release of Xcode which features the macOS 11 SDK, and the major version will be 11 and the minor version 0. For those porting their apps now to Xcode beta-releases, that is again a consistent target to work to.
There will be no difference between Intel and Apple Silicon systems in this respect: the determinant is the SDK, not the target hardware.
Shell and other scripts don’t have the benefit of an SDK to take care of such matters for them. If the calling environment sets the variable SYSTEM_VERSION_COMPAT=1 then Big Sur will return 10.16 as its version number; with that absent or set to 0, the version returned will be 11.0. This should ensure that those who need compatibility with the previous version numbering can obtain it.
Thanks to @jeremyhu via @rosyna for confirming this (these tweets are shown oldest at the bottom):