Cross-cluster search has stricter version compatibility constraints than other features involving remote clusters. For instance, Elasticsearch 8.1 can have 7.17 remote clusters but CCS is not supported on this pair of versions. In practice CCS works ok between 8.1 and 7.17 but we aren't testing this and therefore expect some future 8.x version to become CCS-incompatible with 7.17. It's not clear how such an incompatibility would manifest, but I would guess it could be a wire-format deserialisation failure which is typically painful to track down and causes the whole transport connection to close (see also #75334).

I think we should remove this lenience and actively reject CCS requests to incompatible versions as per the documented compatibility matrix to avoid such a future surprise.