Right now, if you do a search on the front page of manpages.debian.org for interfaces you'll get sent to the copy of man 5 interfaces from ifupdown2. I would imagine that most of the time the user really wants to see the manpage for ifupdown given that a) it is a package installed by default b) Debian's /etc/network/interfaces is unique to Debian and the manpage is the authoritative documentation for it.
I don't know the total web of URLs and redirects available on manpages.debian.org but I would not be surprised if there were some pages to searches linked around on the greater internet that are now getting redirected to the ifupdown2 version of the manpage.
Perhaps it is possible to prioritize the manpages selected for the "I'm Feeling Lucky" search on the front page to prioritize packages that are Priority: important or otherwise included in the default installation of Debian?