Is it possible to make shortcuts available only after an in-app purchase?

I would say no. But others may have another interpretation. The limit is the same for monetized widgets or others extensions. They are available but it is their content that is monetized.

https://developer.apple.com/app-store/review/guidelines/

3.2 Other Business Model Issues
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3.2.2 Unacceptable
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(ii) Monetizing built-in capabilities provided by the hardware or operating system