If you don't need SSL you can't use HTTP/2 or HTTP/3. It doesn't make sense for externally facing services but sometimes is ok or even unavoidable for local networks

It's mostly the property of servers and clients that you interoperate with that don't speak H/2. There are other situations too like if you run a CDN and you're doing sendfile with kernel TLS, you can do stuff like the below

  sock.write('HTTP/1.1 200 OK\r\ncontent-length: ' + f.size() + '\r\n\r\n')
  sendfile(sock, f)

which wouldn't work with H2/H3. WebSockets are still in practice H/1 only (spec exists but is disabled in browsers). Another aspect is that if you're simply using HTTP as a preamble to 1 large (bidi) pipe, the extra framing and flow control of H/2 is just a performance handicap.

Yes, because HTTP/2 in Go is significantly slower: https://github.com/golang/go/issues/47840

believe it or not but Cloudfront only supports HTTP/1 to origin. Some other CDNs as well.

I have clients who have mountains of custom made equipment which only speaks plain HTTP/1.0. Some of it has already been in production for over twenty years and is still doing the job.

While those devices stay static development and updates on the software that interfaces them to the rest of the world has not stopped.

I haven't checked in a while, but I believe nginx does not support http/2 with proxy_pass.

You’re requesting from a third party backend you don’t control?

imo, it's obvious for people writing ajax-y web apps, but backend devs need convincing.