The wasteful SOA query is a result of our internal code flow. You can reduce wasteful queries a bit with the new `consistent-backends` setting, but the SOA query will remain.
We (no longer) have a knob to disable metadata, but there's a cache that should also remember your empty response (the default value for `domain-metadata-cache-ttl` is 60 seconds).
If you have a SOA for example.com, pdns will never return REFUSED for anything inside example.com. The zone is there, so an answer must be present - either a name with records, a name with no records for that type, or the name does not exist. (The distinction between the latter two is why you mostly see ANY queries instead of the type the client asked for).
As far as I can tell, you should never be returning 'false' to anything, as that indicates failure, which is different than 'I know I do not have what you are asking for'. Think of it like this: your SQL database does not go 'oh no' when it has zero rows for you; it just gives you zero rows. A pdns remote backend should behave the same. If you return 'empty' for those lookups into domains you have nothing for, instead of false, I expect REFUSED will come out instead of SERVFAIL.
(And, if I'm correct in the previous paragraph, your PR is correct too :-) )