The fallback DNS was put into place because some users have local DNS that returns non-conforming responses in a way that breaks dns resolution in alpine contai

The fallback DNS was put into place because some users have local DNS that returns non-conforming responses in a way that breaks dns resolution in alpine containers (think broken implementations in some of those shitty router-modem bundles that most ISPs give you). Home Assistant OS is based on containers, many of which run Alpine. They had a lot of bug reports about these issues, and they all went away once the fallback DNS was implemented. They did not originally allow this fallback behavior to be disabled because it should not have been needed, and would only result in those original bug reports returning. However there was a problem in the original design for fallback, where it had an issue where DNS could get stuck always being routed to the fallback servers. Which means local DNS names won't resolve. While annoying, this is often reasonably easy to work around, as one can typically just assign static IPs to relevant local devices and use the IP addresses. They did eventually fix this, so that it cannot get stuck using the fallback as the primary, and local dns names work again. Furthermore, they are now able to detect when locally configured DNS is broken in ways that break alpine containers, so they now offer a feature where you can turn off fallback DNS, but if you have broken local DNS your system will be flagged as unsupported, which means that the devs can summarily close your bug reports without investigating, unless you have reproduced the issue on a supported installation too.