The Mac version of Rise of the Tomb Raider: 20 Year Celebration has now made its way to Steam, while the Linux port will be coming in the near future, though I'm afrad no exact date has been given just yet. In terms of content, the rather ambiguously titled "20 Year Celebration" edition brings with it the base game and all of the DLC in one convenient package. The DLC itself isn't anything special, and a lot of it is cosmetic, but it's still good for a few hours of fun so I'd say it's well worth checking out.
Much like the good chunk of ports these days, the Mac version of the 20 Year Celebration edition has not been made by the developers themselves, but rather by Feral Interactive. You might recognize them from their work on the Mac and Linux ports of Tomb Raider, Total War: Warhammer 1 and 2, XCOM 2, Dawn of War 3, and the list goes on. So far their work has been pretty high quality, so hopefully the same applies in this scenario as well.
While this should be fairly common knowledge for any seasoned gamer, it is also worth mentioning that owning one version of Rise of the Tomb Raider: 20 Year Celebration on Steam will give you access to all three. So if you already own the Windows version, you don't need to pay anything extra in order to access the newly released Mac and upcoming Linux versions.
You can learn more about Rise of the Tomb Raider, as well as read any of the recent user reviews, by heading over to Steam. And just for good measure, allow me to leave you with a brief trailer showing off the 20 Year Celebration edition. Enjoy!