Github user NicoK commented on the issue:
https://github.com/apache/flink/pull/4552
one thing which we talked about offline: as a precaution, we should keep
the old implementation around and allow the users to basically turn the
credit-based flow control algorithm on/off (the accounting for the credits
would mostly stay in that case but will simple not be used by the old
non-existing flow control)---
