Github user pnowojski commented on the issue:
https://github.com/apache/flink/pull/4749
Users shouldn't be able to call `shutdown()` and if they are, they
shouldn't be using it (it would be incorrect). `shutdown()` was a protected
method and shouldn't be used the users. Unfortunately in many places in
`flink-tests` it was used instead of public `stop()`, because protected methods
are visible inside the same package and it was really confusing which one
should be used.
Ideally it would be great to have another visibility level like "only
visible to the children", but there is no such thing :/ Thus I renamed
`shutdown()` to make clear distinction from public `stop()`.
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