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In which context is `--progressive` mode useful?

<https://kotlinlang.org/docs/whatsnew13.html#progressive-mode>

For example, `1.6` will introduce a breaking change for `when` to make it always exhaustive. Would enabling progressive mode in `1.5` allow our code to be cross-compatible with both `1.5` and `1.6` in theory (assuming this change lands as a hidden option in the `1.5` compiler)?

Not answering your question, but FYI the breaking change won't be until 1.7. Just a warning in 1.6.
See the Migration Plan heading in <https://youtrack.jetbrains.com/issue/KT-47709>

Progressive mode turns all warnings for deprecation changes into errors regardless in what language version this error will be reported by default.

Warning for KT-47709 is marked as deprecation, so `--progressive` will turn them all as error too