Many, many sites haven't even started migration to http/2 yet. Why should I care about http/3 now? It seems kind of far in the future.
But this book isn’t about concerning yourself with using it or implementing it, it’s about understanding what the future holds, how it works, and what roadblocks lie ahead.
The lack of API support in OpenSSL for it’s TLS requirements and poor optimization for heavy UDP traffic loads on Linux et al (they say it doubles CPU vs HTTP/2 for the same traffic) sounds like it’s going to be a major hurdle for widespread adoption any time soon.
If they add easy support in NGINX and HTTPD, then its easier for self-hosted endpoints to change as well, with minimal to no effort on their side.
The way I have understood, the book says that what is now in use (these 7%) is a "Google-only-QUIC" whereas the "standardized HTTP/3" is still used... nowhere?
The IETF QUIC remains a work in progress, perhaps to be published in 2019. HTTP/3 is an application layer on top of (IETF) QUIC, it might also be published in 2019 or later. There are implementations of current drafts, and the rough shape is settled but they're a long way from being truly set in stone and aren't in anything ordinary people use.
So unsurprisingly nobody is already doing a thing that isn't even standardised yet, but people are, as you see, writing about it.
"> The book says 7% of all internet traffic already uses QUIC (HTTP/3)"
was wrong: the book doesn't say that, and that what is claimed that the book says (even if it doesn't) is false in more aspects.
IMO it's positive. We are getting free new stuff, and I actually prefer to have two incremental steps, where HTTP 2.0 still uses TCP, giving stuff like multiplexing and pipelining, and HTTP 3.0 uses a novel UDP based transport layer protocol, improving stuff further.
There is objectionable stuff like the recent manifest 3.0 version to make ad blockers crappier but this is not one of the objectionable things imo.