> There is no guarantee the name server they are querying is the same as the server in the A result
That's ok. Let me try to explain a bit more:
Queries to 1.1.1.1 are going over public internet. And even though they are encrypted, they also carry metadata with them, including IP addresses of who is doing them, precise time, rough size, various OS specific stuff, etc. And packets going out to authoritative servers from 1.1.1.1 are in clear text. There is a very tiny window of possible queries out of 1.1.1.1 for encrypted data coming in from some IP address and therefore only a tiny number of possible responses from authoritative servers. Given that and enough intercepted data all over the world it is easy to correlate clear text DNS responses with IP addresses or who got responses from cache and on which popular website ended up, etc.
You seem to misunderstand it. There are less points to intercept traffic at with 1.1.1.1, than without it. Much more feasible to spy on a massive scale, much less privacy and usefulness of client subnet EDNS option completely disappears. In 1.1.1.1 case it's literally irrelevant for privacy whether they do it or not. 1.1.1.1 already hurts privacy massively and not passing client subnet only hurts competing CDNs.
This is no longer a factual discussion. You mention two separate issues:
1. Use of EDNS client subnet information harms user privacy, by providing information that would not otherwise be there.
2. Many users on a single global DNS provider lowers the amount of points that needs to be attacked to obtain DNS information.
However, you position your statement as if #2 somehow render #1 moot, which is an entirely subjective evaluation from the perspective of a user, and also not at all relevant to the discussion of #1, as that on its own is not 1.1.1.1 specific.
For an example of why this is very subjective, the user may believe that the security of ISP DNS servers is likely not trustable, and that infiltrating countless ISP DNS services would likely be much less work than infiltrating one of the larger providers, such as 1.1.1.1, with better security practices.
The only things relevant to this discussion is whether or not it is sensible to respond with bogus data to a valid request that does not contain optional fields, and separately whether or not it is sensible for a DNS provider to not contain these fields.
That's ok. Let me try to explain a bit more:
Queries to 1.1.1.1 are going over public internet. And even though they are encrypted, they also carry metadata with them, including IP addresses of who is doing them, precise time, rough size, various OS specific stuff, etc. And packets going out to authoritative servers from 1.1.1.1 are in clear text. There is a very tiny window of possible queries out of 1.1.1.1 for encrypted data coming in from some IP address and therefore only a tiny number of possible responses from authoritative servers. Given that and enough intercepted data all over the world it is easy to correlate clear text DNS responses with IP addresses or who got responses from cache and on which popular website ended up, etc.