Time for a new messenger, and I don’t mean Signal, but the creation of some kind of old Skype, with a peer to peer protocol. It was very good before Microsoft bought it. Of course, if the code is open and does not require a proprietary server part.

There are no ready-made tools available to automate such deployments. Especially if a certificate must be the same for each of the hosts, fingerprint included. Having a single, authoritative certificate for a domain and its wildcard subdomains deployed everywhere is much simpler to monitor. It does not expose internal subdomains in certificate transparency logs.

Unfortunately, organizations (persons) involved in decisions, do not provide such tools in advance.

You're right, with HTTP/1.1 we have single request in-flight (or none in keep-alive state) at any moment. But that doesn't limit number of simultaneous connections from a single IP address. An attacker could use the whole port space of TCP to create 65535 (theoretically) connections to the server and to send requests to them in parallel. This is a lot, too. In pre-HTTP/2 era this could be mitigated by limiting number of connections per IP address.

In HTTP/2 however, we could have multiple parallel connections with multiple parallel requests at any moment, this is by many orders higher than possible with HTTP/1.x. But the preceeding mitigation could be implemented by applying to the number of requests over all connections per IP address.

I guess, this was overlooked in the implementations or in the protocol itself? Or rather, it is more difficult to apply restrictions because of L7 protocol multiplexing because it's entirely in the userspace?

Added: The diagram in the article ("HTTP/2 Rapid Reset attack" figure) doesn't really explain why this is an attack. In my thinking, as soon as the request is reset, the server resources are expected to be freed, thus not causing exhaustion of them. I think this should be possible in modern async servers.