What libraries are ending support for HTTP/1.1? That seems like an extremely bad move and somewhat contrived.

"5.2. HTTP/2

HTTP/2 [RFC7540] is the minimum RECOMMENDED version of HTTP for use with DoH."

One paper I read some years ago reported DoH is faster than DoT but for multiple queries in single TCP connection outside the browser I find that DoT is faster

I use a local forward proxy for queries with HTTP/2. (Using libnghttp2 is another alternative). In own case (YMMV) HTTP/2 is not signifcantly faster than using HTTP/1.1 pipelining

For me, streaming TCP queries with DoT blows DoH away