You can set up design documents that implement access control logic. These are written in JavaScript.
https://docs.couchdb.org/en/3.2.0/ddocs/ddocs.html (note the design document for the users database)
This isn't the right type of solution for all data - but there's a lot of situations where people have a bug lump of data that they're trying to expose... list of all current flights, or classes, or ongoing games.
It can be quite useful for a lot of use cases. One size never fits all.
