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
An update over the course of the previous 19 hours [1][2]; apparently that foreseeable future ended 10 hours after you posted this excellent reminder. They have begun closing [3] all outstanding github issues for v3 with the text:
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
[2] https://news.ycombinator.com/item?id=12435425
[3] https://github.com/twbs/bootstrap/issues/16913