I have the same issue but the official answer from Apple was "no" - https://developer.apple.com/forums/thread/134115. On the other hand, iOS has the ability to deploy a custom keyboard encapsulated within a top-level application that users install from the App Store. It would be ideal and consistent if Mac OS also supported something similar. From experience, I can tell you that distributing IMEs outside of the App Store is a major pain for both the developer and users.
ratreya
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