Good day,
Did a quick http3 test by adding:
listen 192.168.x.x:443 quic;
add_header Alt-Svc 'h3=":443"; ma=86400';

This will break Matomo/Piwik:
Matomo Version: 4.14.2
PHP Version: 8.2.7
Server Operating System: debian sid
Browser: chromium and firefox same result
nginx version: nginx/1.25.1
built with OpenSSL 3.0.9+quic 30 May 2023

From the debug log...
fastcgi param: "HTTP_HOST: pwk.domain" is missing over quic
fastcgi param: "HTTP_REFERER: https://pwk.domain/index.php?module=Login" (http2) gets changed to
fastcgi param: "HTTP_REFERER: https://pwk.domain/index.php" (quic)

Of course this will also break wordpress or any other app that is using HTTP_HOST ("PHP message: PHP Warning: Undefined array key "HTTP_HOST" in .../wp-includes/admin-bar.php on line 472"). Will also empty nginx var $http_host.

Appreciate any help or pointer

PS: https://github.com/matomo-org/matomo/issues/20916 has some more info.