I just double-checked and it seems like Address review comments on `JSONRPCConnection` update by bnbarham · Pull Request #29 · swiftlang/swift-tools-protocols · GitHub indeed did not make it into the 6.3 release. It only exists in swift-tools-protocols 0.10.0 but 6.3 still references 0.9.0 (swift/utils/update_checkout/update-checkout-config.json at release/6.3 · swiftlang/swift · GitHub).
It should be fixed in the nightly main toolchains though. Sorry for causing the confusion.