It's difficult to look at a release based on Bootstrap 4 without an indication from the Bootstrap 4 project of when they are going to make an official release of their own platform.
Bootstrap Alpha 3 was released 12 days ago so there appears to be renewed interest; this was the first release this year.
By and large the
back-filling-bootstrap-4 branch is good to go bar any changes due to the evolution of the Bootstrap project itself.
We had hoped things might progress a bit faster from Bootstrap -- if we'd known it was going to be 6 months between alpha releases we would probably have started back-filling for the current Bootstrap 3 implementation of Material.