This was a functional bug in 6 - 6450207. The notion of Task completion matches the semantics of Futures where completion is indicated by isDone() and signifies normal completion, exceptional completion or cancellation. This is also indicated elsewhere in the TPE docs eg see afterExecute.
The question of whether this fix then warrants an entry in the compatibility notes is a tricky one. Naturally if this bug has surprised you, you see the change as an incompatibility and would like to see it documented. For others this is just a bug like a dozen others and they don't all need to be documented. I'm in the latter camp but sympathetic to the former. :) I will see how painful it is to make such changes.
Thanks David. Hopefully the facts that we had a testcase specifically checking for the changed behaviour and I took the trouble to report this perceived incompatibility should warrant updating the page.