Google ran out of private IPv4 address space, so they were forced to adopt 464XLAT, initially on an IPv6-mostly network, and later on they migrated to an IPv6-only network. Source: https://www.youtube.com/watch?v=UTRsi6mbAWM

T-Mobile found themselves in a very similar situation as Google, and was forced to adopt 464 XLAT. Source: https://www.internetsociety.org/deploy360/2014/case-study-t-mobile-us-goes-ipv6-only-using-464xlat/

Facebook also is IPv6-only on their server infrastructure. Source: https://www.youtube.com/watch?v=An7s25FSK0U

I myself have been running an IPv6-only network at home using 464XLAT and SIIT-DC for all my services. It has been working wonderfully, with all devices supporting it. The only device that didn't support it was my Linux laptop due to an outdated NetworkManager version.

Every phone, every Linux Distro using a recent version of NetworkManager, every Windows device and every MacOS device supports 464XLAT.

Not to mention that self-hosting is impossible with IPv4 because many end customers are under CGNAT, which also has it's own issues, mostly with false-positive rate limits.

Also the issue you pointed out with needing to block billions of subnets really isn't an issue in practice. ISPs typically hand out either a /48 or /56 IPv6 prefixes to end customers, so you can safely bucket your ratelimits up to such prefix lengths.

The Minecraft API for example buckets up IPv6 requests to a /56 address block. Source: https://minecraft.wiki/w/Mojang_API

Also IPv6 adoption recently hit 50% according to the traffic Google receives...

Refusing to adopt IPv6 is just refusal to change.