So I've given this some more thought, and for a brief moment was considering increasing the log level for this message, but I really think we shouldn't.
Consider the following scenario: your ISP offers proper IPv6 connectivity, your OpenWrt router is announcing an IPv6 prefix to your LAN, and you have working IPv6 connectivity on your clients. One day your ISP messes up and breaks that IPv6 connectivity. In the previous scenario, it would stop working, but no log messages appear in your router, so you assume everything is fine...
You might find out accidentally, but I'd prefer the router being noisy about it.