This means that LTS 4 is being discontinued, and LTS 5 is imminent. See the announcement blog post:
https://unknownparallel.wordpress.com/2016/01/18/stackage-is-reverting-to-aeson-0-9/

See also related discussion (yesterday):
https://www.reddit.com/r/haskell/comments/41gpdk/lts4_with_aeson010_is_being_discontinued_lts5/

If there are any questions or concerns, don't hesitate to ping me.

-- Dan Burton