Earlier quoted context omitted.
Right now we don't have a maintained, release version of bootstrap. That's not really reasonable at all, imo. I understand wanting to focus on the next version, but maybe continue to maintain v3 until v4 comes out? Ditching support the day v4 drops is still kinda premature, imo, but still better than this limbo we're left with. Frankly this seems incredibly unprofessional.
The maintainers of Bootstrap have day jobs. Bootstrap is not the day job. Insisting someone maintain your idea of professionalism in what is essentially a public service they perform in their free time is an odd way to spend your free time. You want bugs fixed on v3? Get off of HN and fix 'em.
IMO, the "not your vendor" line is perfectly reasonable when we are talking about code that was just made available by posting it in a GitHub repo or a blog. Once the members of a project actively promote their product to other people, and happily witness very large numbers of people take the advice and use the product for years in their own projects, I think that it's rather a breach of trust to suddenly change their minds.
The minimum that could have done is given notice that they were struggling for resources and given people in the community an opportunity to help, and perhaps even request that someone from the community step up and become Bootstrap 3 maintainer to handle any necessary bugfixes until users could transition to Bootstrap 4.