This is now solved with a new API in iOS 15. https://developer.apple.com/documentation/uikit/uideferredmenuelement/3857602-elementwithuncachedprovider?language=objc

I got an answer to this elsewhere. The enabling or disabling of the Siri capability (and others) is actually stored in the project's Entitlements file. So I duplicated my existing Entitlements file, removed Siri from it, and set that Entitlements file as a variation for the macOS SDK in the project's "Code Signing Entitlements" build setting.

I actually figured this out with the App Store Connect UI. In my scenario above, when adding the introductory offer, it displays "every 6 months for 12 months". I'd attach a screenshot here if the forums allowed images.