Originally published via community forum on 27 September 2017.
In recent years backwards compatibility has been a priority for various different teams at Apple and one of them is the Developer Tools team. Together with the macOS team they appear to have started synchronizing releases allowing the developer community to find, report and help Apple fix issues with new releases of both macOS and Xcode.
This chart on Wikipedia shows that across the last 17 Xcode releases an operating system upgrade was required roughly every 6 months or 165 days for the latest and greatest Xcode version.
The breaking releases usually happened around March and September and if you look at events on Apple’s yearly release calendar those dates are spot on in September with new major OS releases. The March releases leave some room for interpretation though. One version could be that new feature development on Xcode stops at that point in order to button everything up for WWDC in June. Looking at Twitter this seems to line up with the “no more features” theory since the first tweets with #nosleeptildubdub appear around end of February every year.
Looking out into the future of macOS 10.13 High Sierra and Xcode 9 backwards compatibility it looks like a major upgrade will be required in March of 2018 or 196 days from now.
Depending on your development and release schedule we would recommend to wait until at least November or even later. This will allow for the Apple community to pick apart every last inch of High Sierra and find all the trivial bugs that turn into weeks of broken builds and endless configuration guesses in a CI setup.