From Fedora Project Wiki

Disable systemd-oomd requires more than disabling it to get rid of it.

systemd-oomd is a background service that kills your programs when you're trying to do soemeting. With systemd, it's not enought to disable them, since other services will start it regardless that it has been disabled. Systemd has thing called masking that will anyone from starting the given service.

# systemctl mask systemd-oomd
Created symlink /etc/systemd/system/systemd-oomd.service → /dev/null.

After masking it also disappears from systemctl --type=service listing. Direct query shows its state:

# systemctl status systemd-oomd
○ systemd-oomd.service
    Loaded: masked (Reason: Unit systemd-oomd.service is masked.)
    Active: inactive (dead) since Wed 2021-09-08 14:14:59 EEST; 12min ago
  Main PID: 2573 (code=exited, status=0/SUCCESS)
    Status: "Shutting down..."
       CPU: 20min 18.627s