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.
Syntax here is not anything like a well thought out proposal, just a demonstration of what I believe to be a shell agnostic method of describing both 'where can I complete?' and 'how can I compete it?'
By 'declarative' I mean some way of telling the shell how my bin can provide completions itself.
E.g. for the AWS util, I am envisioning something like
Syntax here is not anything like a well thought out proposal, just a demonstration of what I believe to be a shell agnostic method of describing both 'where can I complete?' and 'how can I compete it?'