Currently, the Xcode app cipd package needs to have older runtimes manually copied into the app bundle to support watchOS and older iOS simulators. This makes crafting and testing new Xcode app bundles very painful, error-prone, and take forever to upload. We also have a custom flutter_mac_toolchain forked binary to prevent the watchOS runtime from being filtered out. I think we only have that binary and don't even have the changes that we made tracked in any repo?

The default depot_tools mac_toolchain now supports separate Xcode and runtime cipd packages.
See docs at https://g3doc.corp.google.com/company/teams/chrome/ios/docs/infra/xcode_upload.md#upload-ios-runtimes. See also discussion in https://bugs.chromium.org/p/chromium/issues/detail?id=1225115#c4

Adopt these new runtime cipd packages to make adding new simulators and new Xcode versions easier. It also means (for iOS, not watchOS) we can use https://chrome-infra-packages.appspot.com/p/infra_internal/ios/xcode/ios_runtime/+/ instead of maintaining our own. AND it means different builders can request different runtimes (web engine builder needs different simulator versions than the iOS engine builder, for example)