HTTP/3の利用は順調に増えてるけど、(end-to-endでの)IPv6の利用も順調に増えてるから
Cloudflareさんによれば
日本全体: IPv6 47.5%, HTTP/3 30.7%
BIGLOBE: IPv6 70.7%, HTTP/3 34.5%
だそうな

日本
https://radar.cloudflare.com/jp
https://radar.cloudflare.com/adoption-and-usage/jp
BIGLOBE (AS2518)
https://radar.cloudflare.com/as2518
https://radar.cloudflare.com/adoption-and-usage/as2518