The Bootstrap maintainers decided to perform that service, defined the terms of the service was, encouraged people to use that service, and now appear to be withdrawing that service at no notice.
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.
