Matz has talked about it as an opt-in. I think his opinion is to not force people to adopt it if they won't enjoy it or their productivity will be hampered by it.

Was at the Kaigi and on the opening keynote Matz talked a bit about it. He doesn't really support adding it since he feels like in 20/30+ years, advances in technology will make writing types "obsolete".

It's a weird argument. If it's not necessary in 30 years (I doubt it because some of the type inference issues are simply undecidable, and that in much simpler languages already), then the language can just opt to ignore type annotations or merely check their consistency. Having them is great either way for the benefit of documentation.

My take is he doesn't really like (or "get") types and feels protective of his language, which is fair. I think Python made an interesting choice here, i.e. provide syntax for types without semantics. The drawback there is potential fragmentation on type system semantics.