afaik it's planned, but it's not a recent suggestion and there's no eta on it.
if you don't need screenshot/cover previews from the latest updates page, currently you can use forum's advanced search to search the first posts of threads in the games subforum for "linux" or "mac" + any tags/prefixes you want. otherwise you could filter by engines known to be cross-platform (e.g. renpy), use wine and such.
for games supporting android or ios, there's mobile game tag (which can be either - you can further narrow it down by using advanced search, this time combining the tag with either android or ios for the keyword)
operating systems listed in game posts is just a thread template, filled by uploaders / thread creators. not all threads use "os: windows, mac, linux, android" yet (you can still find ones with "plaftorm:" or different spellings of systems, and sometimes typos).