> I don't understand how "using an index" is a solution to this problem. If you're doing search, then you already have an index

It's a separate index.

You store document access rules in the metadata. These metadata fields can be indexed and then use as a pre-filter before the vector search.

> I don't think Elastic supports this

https://www.elastic.co/docs/solutions/search/vector/knn#knn-...