The most active multiplayer Doom port is Zandronum, using the Doomseeker server browser to join games. Mac support may be a bit flaky, but I think it currently works (join the Discord for help).

https://zandronum.com/ - get the software, join the discord

https://doomlist.net/ - see active servers

I've always much preferred Zandronum over Zdaemon because it's open source and it has much more advanced mod capabilities (with much more Zdoom/Gzdoom mod support than Zdaemon), supporting mods adding game modes like the PVE Invasion mode.

There is the community around ZDeamon [1]. As a Doom port, it has its own server browser, Discord and a built-in IRC browser to get to their channel. Binarys are available for Win, Linux and MacOs. IME, especially popular there are gigantic megaWADs, which are played by fifteensomething players in coop mode simultaneously.

[1] https://www.zdaemon.org

Holy cow, 15 players in coop sounds really fun!

As long as a game allows someone to run their own server, a community never truly dies. There are people who still play Half Life 2: Deathmatch and Counter Strike Source because we can run our own server

>Counter Strike Source

Ha. There's still people on CS1.6. I dip in a play a couple games every now and again. I found a server, not too long ago, that was running a full conversion Rocket League mode. Occasionally, you could see a flash showing that you're just a CT/T wielding a knife before the custom graphics load in and you're playing soccer with rocket propelled cars.

CS 1.6 can be played on a browser nowadays.

I have always considered Quake to be the natural successor to Doom, and it is fully multi-player.

https://en.wikipedia.org/wiki/Quake_(video_game)

Zandronum if you want to use mods, odamex if you prefer a vanilla experience. The odamex discord server is a good starting point.

There is also https://www.zdaemon.org/?CMD=downloads

Doomworld forums are a good place to find the current doom community folks.