Then why say that you're ending the support for Bootstrap 3? Clearly something IS not going to be done from this point onward, and it's that thing which is worrying people.
At this point "ending the support" is a semantics fight. When the Bootstrap team here says they are ending the support on v3, they are saying that they are closing issues and PRs that are v3-only and focus on finishing v4.
What they are not "ending" at all is "We still recommend v3 for production and believe it to be stable".
The complication here is the two meanings of "support" of "recommendation to use for your production efforts" versus "we are actively helping a tiny fraction of users with long-tail bug reports".
Then why say that you're ending the support for Bootstrap 3? Clearly something IS not going to be done from this point onward, and it's that thing which is worrying people.