Doom Eternal's latest update now include Denuvo Anti-Cheat, not to be confused with Denuvo Anti-Tamper. Unlike Anti-Tamper, Anti-Cheat runs at the ring-0 kernel level of your PC.
The Anti-Cheat can be uninstalled, however it is reportedly required to run Doom Eternal, even in singleplayer mode.
One consequence of this new anti-cheat software is that it is now incompatible with Proton (Linux) and its ProtonDB rating has fallen from a previous Platinum award - to completely unplayable.
Irdeto, the company that maintains Denuvo, has been working on this software for quite some time and it appears that Doom Eternal is the first game to use this software. This announcement about the software was made back in March 20, 2019:
In this Doom Eternal announcement of May 14, 2020, they state:
Check out our Denuvo wiki article which has been updated with information about Denuvo Anti-Cheat.