Official info (currently only on Czech but will be published in English) is locatd here https://blog.nic.cz/2021/10/18/25-rijna-zkusebne-vypneme-ipv6-tunelovaci-technologie-teredo-a-6to4/ (https://blog.nic.cz/2021/10/18/25-rijna-zkusebne-vypneme-ipv6-tunelovaci-technologie-teredo-a-6to4/)
Citation from the article:
QuoteBased on the above, we decided to turn off both IPv6 tunneling technologies for testing on October 25, 2021 for 24 hours.
If no major problem arises, we will definitively terminate the operation of both services on Monday, November 1, 2021.