While one can debate if a "guest" WLAN should have the privilege of IPv6 connectivity, I think the guiding principle here would be to avoid the inevitable "Why does [device X] not work on my guest network?" because it somehow got an IPv6 address, prefers it, and fails. Easier to disable IPv6 at the firewall for a zone than it is to enable it.
We are going to see more and more ISPs with ipv6 only deployments, particularly WISPs, also DSlite deployments are common in Germany.
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