The example of running Pigeon available here, mentions that Swift and Kotlin generation are experimental.

A flag for Swift generation is included in the example (experimental_swift_out), but Kotlin ones are not. Moreover, I couldn't find the exact name for the experimental Kotlin generation flag without looking at Pigeon's source code. In fact, the section Swift / Kotlin Plugin Example links an example project that still hasn't the Kotlin flag in its run_pigeon.sh.