The latest working public Teredo relay (the one which relays traffic to any IPv6-enabled host from Teredo address), TREX.fi, doesn’t seem to provide Teredo service anymore.
The teredo.trex.fi address does not ping, does not reply to Teredo packets.

There used to be teredo.remlab.net from miredo software author, but it stopped its operation in 2021 and domain has been rerouted to TREX IP address. Now it does not resolve at all, after I emailed the author with the news.

The only still working Teredo server known to me is Microsoft’s win10.ipv6.microsoft.com and win1910.ipv6.microsoft.com (resolves to different IPs), but it does not provide the relay, and thus does not allow to access IPv6 services over it. It is basically provided for for NAT punching in Xbox games and connectivity inside Teredo network.

Перед закатом появился teredo.august.tw. Анонсы префикса до сих пор идут, но сам сервер не отвечает. Можно спросить, там и телеграм в контактах есть.

False alarm: teredo.iks-jena.de server works fine and has a working relay.
For some reason, I always thought it’s long gone, but apparently it still working fine.

The Great Firewall of China spoofs DNS AAAA replies using 2001::/32 Teredo address range. The number of IPv6 users is growing in China now, so Teredo relays might be overloaded in result.