They mention in the original alpha release article this was a mistake. They plan on supporting and bug fixing for the foreseeable future with bootstrap3
Bootstrap 3 is no longer being officially developed
or supported.
All work has moved onto our next major release, v4.
As such, this issue or pull request is being closed
as a "won't fix." For additional help and support,
we recommend utilizing our community resources.
Thanks for your understanding, and see you on the
other side of v4!
<3,
@mdo and team