With the new lambda syntax there was growing support for completely replacing string lambdas with the new syntax.  I got ahold of Andrei and asked him whether std.algorithm and std.range should continue to use them in their documentation.  He got back to me at some point (presumably after discussing with Walter) and gave me the go ahead to switch the documentation examples over to the new style of lambda (while retaining support for string lambdas for backwards compatibility).  This resulted in #707 which didn't get pulled (well, it did but got reverted because of a dmd bug) and then bitrot but the decision to retire string lambdas in phobos has been made by Andrei and Walter.

I think the discussion in #707 is probably the extent of the public discussion about no longer using them.  My original conversation with Andrei took place on IRC.