In a bit of a surprising move, Bootstrap v4 will be dropping support for IE8. Those who need IE8 support can continue to use Bootstrap v3.x. According to at least one dev, Bootstrap will continue to provide bug fixes/patches for v3 when needed. How long this will last is anyones guess.
I can appreciate wanting to move forward, but in my own personal experience, many small to medium businesses and clients require IE8 support for legacy computers, users etc. I suppose it all depends on your user base, and Bootstrap feels comfortable enough with IE8 numbers to do so. More power to them, but for many in the corporate world v3 will be the only choice for now.
A few resources where you can read more about this change: