The links from that post and top comment point out that that initiative was dropped. It got mired down in bikeshedding from hundreds of opinions and SO eventually just said, "Fuck it."

The MIT announcement thread was edited with the cancellation announcment:

Update: January 15, 2016

Thank you for your patience and feedback. The changes proposed here have been delayed indefinitely - we'll be back later to open some more discussions.

The top comment from your link points out the current license:

TL;DR: Source code on SO is still licensed under CC-BY-SA.

And CC BY-SA is the only license listed on the official help page.

• Content contributed before 2011-04-08 (UTC) is distributed under the terms of CC BY-SA 2.5.
• Content contributed from 2011-04-08 up to but not including 2018-05-02 (UTC) is distributed under the terms of CC BY-SA 3.0.
• Content contributed on or after 2018-05-02 (UTC) is distributed under the terms of CC BY-SA 4.0.