Well... OpenWrt's upcoming version 21.02 now defaults to setting up global ipv6 routing. If you don't have an ipv6 delegation connection, well that's fine, OpenWrt will set up a private ULA network. Then still claim it's got global routing, killing every ipv6 device's networking.
Most devices will fail back to IPv4 networking once they realise IPv6 is broken, but you'll find lots of little things broken. For instance, Home Kit hubs getting too confused to talk to each other.