When reviewing a revision on the deleted Flow board (e.g. http://en.wikipedia.beta.wmflabs.org/w/index.php?title=Special:Undelete&target=Talk%3AET56&timestamp=20151015210301)
after clicking in the edit area, a user receives a misleading warning:

Using a permalink for the topic on the deleted Flow board, a user may type some text and only upon saving receive

Note: insufficient/misleading user warning is also present in some cases when Flow boards are protected. See my comments on T108762: Notify users who are editing protected flow content.