Hello Salvatore!
Unofficial and amd64 only builds (including the source in case you
want to built it on your own) are at:
https://people.debian.org/~carnil/tmp/varnish/
I've installed v6.1.1 packages on several of our Buster servers.
Apparently all the websites and portals hosted there are feeling well. I
tested access with HTTP2 as well as HTTP 1.1 only. Also continuously
firing 100 req/sec with locust against this patched Varnish works fine.
Shall I test the packages on Bullseye too (could do that on Monday), or
is Buster already enough?
Cheers,
Andreas