The new release of nss-mdns 0.11 implements the important heuristics of disabling itself if there is an authoritative unicast DNS SOA response for .local, or if the request has more than two labels (like "X.Y.local").
These heuristics are disabled if /etc/mdns.allow is found. Gentoo should probably not install this file by default anymore, and remove it if the old config file was never changed after installation.
If this is not possible, we could change nss-mdns to support this functionality in another way, but hopefully this is not necessary.
See https://github.com/lathiat/nss-mdns#documentation (search for "SOA" on that page).
You probably want to also update the pkg_postinst message to talk about the new functionality and recommend that users do not create the /etc/mdns.allow file.
Note also that the file is completely ignored (and the heuristics always active) with the _minimal variants.