SixXS will be sunset in H1 2017. All services will be turned down on 2017-06-06, after which the SixXS project will be retired. Users will no longer be able to use their IPv6 tunnels or subnets after this date, and are required to obtain IPv6 connectivity elsewhere, primarily with their Internet service provider.
If you are still using a 6in4 tunnel from it, start looking for alternatives.
EDIT:
based on SixXS statistics, about 40% decrease in the traffic since the sunset announcement. But looks like half of the users are still there. So, likely there are still also LEDE/Openwrt users...
Any alternative for us poor people without a public IPv4 address?? SixXS worked without. I signed up to HE, but I can't get passed the IPv4 endpoint. How to get this working?