Getting onto H2 is worth investing in then though, and drastically simplifies things that would otherwise need bulk operations everywhere.

I'm not entirely sure what you mean with downstream, but in my experience H2 is really well supported almost everywhere these days. Ymmv ofc

What I mean is, typically, there are many layers between an end user's network request and reaching the server where it will be fulfilled. For example, you might have your site fronted by Cloudflare. They provide DDOS protection, etc. That Cloudflare server might accept HTTP/2 requests, and then open a new request to your load balancer server. Then, your load balancer opens another network request to the server fulfilling the request. The connections between these servers may only support HTTP/1.1. So you lose the benefits of HTTP/2 since you can't multiplex the network request end-to-end.

In this particular example (S3), I don't think this is really a concern.

Anyway, I'm curious... what components do you run into that don't support this yet? At least in your examples I can't think of any major players that don't do HTTP/2 out of the box, except, well, old versions.

Is this more of a hypothetical? In my experience HTTP/2 is pretty much ubiquitous for anything current.

There are environments that do not allow http2 or websocket traffic on the network. Most government/big enterprise have these restrictions.

The load balancer could append a header though with a session ID that is the same for the same HTTP/2 session, and then the server can batch based on the session ID and timeframe.