It seems Feral Interactive have a few less ports available to buy supported for Linux and macOS to start off 2021 with as both Mad Max and Shadow of Mordor have been delisted for both platforms.
The change happened just before the end of 2020 on both titles, as seen on SteamDB (#1 - #2). On the macOS side, they lost even more as a few Lego titles also vanished and Batman: Arkham City too from mentioning macOS. Why? They all have a common publisher - Warner. Confirming this to me on Twitter, Feral Interactive stated "Hi, these games have been removed from sale on macOS/Linux due to their licenses expiring.".
So what does this actually mean? The Linux ports still exist, still work as expected and anyone who buys them on Steam would still get them too as they're attached to standard purchases. Going forward though, they're not being advertised or supported.
A shame but actually somewhat common. We see this with racing games, football games, and sometimes music also causes issues and more where licenses are often for a specific time period. In this case it's a bit more unique that it only affected the external ports.