Is there a wrap-up somewhere that shows how this would be used by clients? The link only leads to some command classes as far as I can tell
For querying data REST APIs used to work quite well already. Using some DSL (like FlowQuery) makes sense for complex things IMO and as long as that’s encapsulated in some query parameters I don’t see a problem with this, e.g. /api/nodes?filter=<some-flow-query>