Last year's Doom reboot had 3 multiplayer DLCs, all of which are now free. Id Software and Bethesda have restructured the game's multiplayer progression system. This also means that players who previously passed on paying for DLC now have free access to the content.
The DOOM 2016 reboot was a single player triumph and a multiplayer failure according to most critics and a lot of players. Bethesda and Id Software were warned about problems with the multiplayer during beta testing, but DLC plans were most likely already in place at that point. Whether DOOM's 3 DLC packs fixed the issues plaguing multiplayer is somewhat debatable.
Either way, the games most recent 6.66 update will be making all the additional DLC content free to all players, and restructuring the mode's progression system.
"The revised progression system will alter when and how players unlock additional guns, demons, equipment, and all of the player and weapon customization options. Instead of relying on the random distribution of unlocks, items now unlock through specific leveling requirements or through in-game challenges that the player can complete", was related by Bethesda in the official announcement accompanying the update.
All progress made by players prior to the update will be reset, but unlocked items and the like will still be available to all who already own them.
In order to incentivise some more people to buy DOOM or join in on the multiplayer, the game will have a free trial running over the weekend, with the usual your progress will transfer into the purchased copy stuff.
Everyone who actually paid money for the DLC, gets a badge.