Is the many worlds theory alive and being worked upon?

Not directly. There's a collection of ideas like the many worlds or Copenhagen interpretations of quantum mechanics, the multiverse aspect of inflationary cosmology, parallel universes from superstring and M-brane theories, and so on. None is a fundamental concept, but more like an aspect or side-effect of a theory. Science is very actively pursuing theories in all of these related areas. But the multiverse ideas are not driving that. One reason is that it is very unlikely ever to be testable.