You can definitely add WidgetKit widgets to an app that supports iOS 13; the system just won't call the WidgetKit extension on older versions of the OS.
bjhomer
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The list of tags on that DevForums FAQ is useful. I wish it were prominently linked from the forum home page itself; it would serve as a great introduction to the topics available on the forum, and might help people find their way to the tags they're looking for.