If you don't use it at all, they do take the prefixes away. I lost a prefix when I did a house move and a server move and took a couple years to get around to setting up the tunnel... They didn't undelegate the DNS until I poked them to either please let me have the prefix back or fix DNS.

I have native ipv6 at home and on my hosting, so that's nice. Unfortunately the he.net tunnels are used for a lot of abuse now and you treated like a bot if you use them to go to popular websites. :(

That would be my assumption, although I wouldn't bet on it being as stable as a traditional domain registration