I can't speak for them, but it's my impression from using 1Password for a good few years (both the local-vault product, and then the "account" subscription service) that local vaults are basically deprecated, even though they work fine. They're just not a good way for AgileBits to make money. So they'll keep them working in the software for existing customers who paid for them and expected them to work; but they won't add new features to them (except by coincidence as part of architecture-level updates) and won't bring them to new platforms where they weren't originally promised to work. They're a legacy feature, serving legacy customers.

For the same reason that they won't bring local vaults to Linux, I don't think they'll ever kill local vaults for macOS or Windows. There are customers who paid for that product, and expect it to still work. (And, unlike e.g. an old version of Photoshop, it's implicit in the USP of a "password manager" product that it'll continue to get updated so that it works on new OSes and so forth, so that you can still have access to your passwords. You can't just stop supporting it; that'd break the whole value-prop of the product, retroactively, and so break the trust of future customers in any "password manager" products you have today.)