[ebbv]

Because it's not meant for you to upgrade an existing front end from Bootstrap 3 to Bootstrap 4. It's meant for starting a new project on Bootstrap 4.

If you already have a working project on Bootstrap 3 why bother upgrading?

If they had to maintain backwards compatibility they wouldn't be able to make a lot of the improvements they make.

[Bahamut]

We have been in the process of upgrading from Bootstrap 3 to 4 at work (in addition to other changes) - libraries we use on top of Bootstrap are moving to 4, so we need to too as long as we continue to maintain these applications.

In addition, it allows us to simplify CSS some by moving away from Bootstrap's crufty float-based grid system into a flexbox system, which simplifies some of our code.

The Bootstrap 3 to 4 upgrade isn't too bad as far as upgrades go.

[niftich]

Presumably because at some point Bootstrap 3 will stop getting bug fixes. Historically, what has been their end-of-life policy?

[niftich]

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.

[yRetsyM]

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

http://blog.getbootstrap.com/2015/08/19/bootstrap-4-alpha/

[niftich]

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

[1] https://github.com/twbs/bootstrap/issues/20631

[2] https://news.ycombinator.com/item?id=12435425

[3] https://github.com/twbs/bootstrap/issues/16913