So based on their github and some searching with google, historically they didn't have much of an end-of-life policy, because there were no bugfix releases for v1.x.x after v2 came out, and no bugfix releases for v2.x.x after v3 came out:
v3.0.0 - 2013-08-19
v3.0.0 RC1 - 2013-07-27
v2.3.2 - 2013-07-27
v2.3.1 - 2013-03-01
v2.0.0 - 2012-01-31
v1.4.0 - 2011-11-03
v1.3.0 - 2011-09-16
With major version 4 still being in alpha, there are concurrent releases of 3 going on still, but based on their previous releases, I don't expect a v3 bugfix release after 4.0.0.
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
v3.0.0 - 2013-08-19
v3.0.0 RC1 - 2013-07-27
v2.3.2 - 2013-07-27
v2.3.1 - 2013-03-01
v2.0.0 - 2012-01-31
v1.4.0 - 2011-11-03
v1.3.0 - 2011-09-16
With major version 4 still being in alpha, there are concurrent releases of 3 going on still, but based on their previous releases, I don't expect a v3 bugfix release after 4.0.0.