How much deployment of servers supporting it is there, but also how much querying from clients is happening? Any sense of the adoption/use trends?

All of the RIRs have RDAP servers, all nominally for testing but in fact working pretty well give or take nits. In Yokahama we went through a list of comments on ICANN's rules for RDAP servers so I expect we'll be seeing gTLD servers pretty soon. R's, John