It seems that years ago the dpkg maintainers strongly disagreed with the "/usr merge" policy, or at least the way it was being implemented using symlinks such as /bin→usr/bin.

They lost that argument, because even Debian itself now requires this configuration. The warning has been disabled for Debian and Ubuntu, but I assume you are running Raspberry Pi OS 32bit, which is based on Raspbian.

(The warning is in /var/lib/dpkg/info/dpkg.postinst, which only runs after dpkg itself is updated.)