There used to be novelty accounts with scores of 100% (achieved via opponent selection). I'm pretty sure any top player right now could get a 90%+ winrate if they played average opponents on Quickmatch/auto-games.
https://www.warzone.com/Forum/28912-highest-win-rate Stronger auto-gamers (e.g., HotBeachBum -
https://www.warzone.com/Profile?p=6341621153) had gotten 80%+