The "declarative" part doesn't really make sense IMO, but there are a bunch of links here about having a shell-agnostic system:

https://github.com/oilshell/oil/wiki/Shell-Autocompletion

https://github.com/oilshell/oil/wiki/Projects-Already-Doing-...

https://github.com/mbrubeck/compleat (one of the only grammar-based ones, looks nice but doesn't appear to have been used very much)

There are some discussions about cross-shell autocompletion on the #shell-autocompletion channel of https://oilshell.zulipchat.com/, but it didn't go that far.

The Ion work described in the sibling blog post went the furthest in terms of implementation I think, but I haven't heard anything about it in awhile.