[MallocVoidstar]

According to the RFC:

>The messages in classic UDP-based DNS [RFC1035] are inherently unordered and have low overhead. A competitive HTTP transport needs to support reordering, parallelism, priority, and header compression to achieve similar performance. Those features were introduced to HTTP in HTTP/2 [RFC7540]. Earlier versions of HTTP are capable of conveying the semantic requirements of DoH but may result in very poor performance.

I'd bet basically all their clients are using HTTP/2 and they don't see the point in maintaining a worse version just for compatibility with clients that barely exist.