Not to be confused with Denuvo Anti-Tamper, which is what we're usually referring to when we mention Denuvo, this is a new "anti-cheat solution" that Irdeto has been developing over the last three years, specifically to tackle cheating in multiplayer modes.
You might not have noticed it if you've played since the update. There's no tray icon or splash screen, and even Irdeto acknowledges that "this invisibility could raise some eyebrows".
When you start Doom Eternal for the first time, Denuvo Anti-Cheat will install a kernel-mode driver that starts when the game launches and stops whenever the game does. Apparently it avoids collecting "any personally identifiable information", and doesn't scan your files, take screenshots or stream shellcode. It does collect information on how your OS interacts with Doom, however, which it sends to Amazon servers.
You're still giving it a lot of access to your PC, though, because kernal-mode drivers hang out at the core of your operating system. There are concerns that this represents a major security risk if someone was to find a vulnerability in it, netting themselves control over the most basic functions of your PC.
Since the update, Doom Eternal has seen a bump in negative Steam reviews—267 so far today—and it's faring no better in the forums. On Reddit, one of the top posts (via Nibel) advises players to remove it from their PCs. Players also chimed in with horror stories about performance problems and crashes.
Riot's been facing a similar backlash for its own kernel-mode anti-cheat software in Valorant, though its version seems to be stricter than Denuvo. It's hard to tell if it's had a positive impact, but Riot did recently ban 9,000 cheaters.