When I used a small local ISP that did not support ipv6 before switching to AT&T fiber¹ I tried to set this up, but they demand an email on a non-gmail domain, and I wasn't going to pay to set that up nor was I going to use my work email. It's a bad assumption that any non-malicious user cares enough about websites to have one.
1: I'd prefer to have stayed with the local ISP despite the lack of ipv6, but they wanted $8,000 to bring fiber to my new place and that was not worth it with at&t fiber being present.
Gmail is a cesspool, and Google couldn't give the slightest bit of a shit. So does it really surprise you that people who share free services might not want to give those free services to people who use the cesspool service that doesn't care about abuse?
GMail is the most popular email provider by a wide margin. Denying service to the largest cohort of email users is indeed surprising, ridiculous, and self-defeating.
On the contrary: it's a decent filter for folks who would be very unlikely to be able to follow the instructions to set up and maintain their end of the tunnel.
(It also happens to be a fantastic filter for spammers and other abusers. Is it perfect? Hell no. But it is very good.)
I can setup email on my own domain, but I don't want to. It isn't worth my time or money. This doesn't mean that I can't follow tunnel broker instructions.
Gmail rejects mailing list messages that every other email provider accepts. It is impossible to get a human at Google to look at these issues. That's kinda the worst of all worlds. At least other email providing companies have support teams that can be talked to when things go off the rails.
Hurricane Electric (for one) offers IPv6 tunnels:
* https://ipv6.he.net
You can configure it on your router:
* https://openwrt.org/docs/guide-user/network/ipv6/ipv6_henet
* https://docs.netgate.com/pfsense/en/latest/recipes/ipv6-tunn...
* https://docs.opnsense.org/manual/how-tos/ipv6_tunnelbroker.h...
Or an individual host:
* https://wiki.archlinux.org/title/IPv6_tunnel_broker_setup
* https://docs.rockylinux.org/guides/network/hurricane_electri...
* https://genneko.github.io/playing-with-bsd/networking/freebs...