The situation

Fast forward 3 months after installing this year's Apple betas.

Final version of Xcode 27 is out.
Just like the beta, it refuses to compile projects with deployment target set to a system that isn't officially supported anymore.

foobar2000 for iOS

Apple has just forced me to quit supporting iOS 12, so all usable foobar2000 mobile branches are OK on new Xcode.

foobar2000 for Mac

Master branch is now for macOS 14 and newer, experimenting with SwfitUI use.

Release branches (2.25, 2.26) target macOS 11 and cannot be compiled in Xcode 27.

Sorting it all out

I still require Xcode 26 for daily work.

For compiling releases, there's xcode-select and xcodebuild, which bypass arbitrary compatibility checks and happily run old toolchains on new macOS.

However, I still need the Xcode app itself to do editing and debugging.

There are workarounds for running old Xcode app on new macOS, most notably launching Xcode binary directly from terminal bypassing the app bundle.
This actually works OK for the most part, some features of Xcode 26.3 crashed on macOS 27, no problems with Xcode 26.6 yet.
Sadly I don't consider this acceptable for main development system, because it sounds like it could stop working anytime with some update.

In other words, the development system can't be upgraded past macOS 26.

... the ugly

To add insult to injury, Xcode 27 drops support for UIDesignRequiresCompatibility flag.

Using this flag suppresses glass effects for your app, making it look as if it was running on macOS older than 26.

The flag has not been removed from new macOS (apps built using older Xcode work as they did before), only the new Xcode disregards it.

The reason why I care is that enabling of glass effects in foobar2000 does THIS:

The problem seems to be well known and widely documented on the internet, yet I couldn't find a clean enough solution for it; this bug is present in multiple table views all over the app.
I'm not going to release foobar2000 with liquid glass enabled until this is somehow fixed.