Here is my input to this.
First of, we have in IPv6 two competing auto-config mechanisms: RA
& DHCP. For a subset of parameters, they more or less overlap. For
others, they do not.
DNS is in the later category, only in the standard track for DHCP.
An example of what can only be discovered with RA is default router
or prefix info.
This has been a very poisonous discussion for many years, with the
result that the IPv6 autoconf story is more complicated that the
IPv4 one.
At this point, I would favor opening the larger discussion of “How
can we fix this mess” rather than pushing a particular technique
from experimental to standard track.
It might be that the only acceptable answer is we need to defined
BOTH mechanisms for every value to discover.