Agreed, if we drop the CI for lower versions, there is actually no point of having safeguards as we can't really test for them.
Maybe one more thought (it's more of a feeling), I feel that users running really old Hadoop versions are usually slower to adopt (they most likely use what the current HDP / CDH version they use offers) and they are less likely to use Flink 1.15 any time soon, but I don't have any strong data to support this. D.
