Multiverse only started becoming mainstream just a few years ago. It was always a fringe idea from the 1950s when it first came out. It inspired a LOT of Science Fiction stories from the 50s, 60s, 70s, 80s, 90s in the meantime.
I’d say it’s only been in the past 7-8 years that it has become seriously discussed, mostly due to the efforts of a few very public cosmologists, the Science Channel and a huge nod from the BBC.
Even still, among friends of mine who are quantum physicists, both theoretical and practical (atom smashers), they just kinda shake their heads at the whole thing.
Multiverse within qm just like other concepts such as anti-time, are primarily utilized as a convenient mathematical device. They’re not stating “This is how the universe really is”. What they’re doing is borrowing a little from impossible things, which are acting as substitutes for other unknowns, in order to make the math work and help provide new insight.
That’s not to say it might not be true. It could be.