With the restriction of 200 char limit it is possible that upgrading a pre 6.6.5 cluster will fail where previously it would have passed.
The reason is that with a key length > 250 in a mcbp response can result in a disconnect and hence a rebalance failure. However it is possible to have a dcp connection name length > 200 without triggering the key length > 250 response issue.
See customer CBSE linked as an example case.
The fix is to remove the restriction by reverting the associated patch https://review.couchbase.org/c/kv_engine/+/114481