https://www.nba.com/stats/players/isolation?PerMode=Totals&TypeGrouping=defensive&dir=A&sort=PPP
Keegan is at the 92.6th percentile in defending isolation in terms of points per possession allowed at 0.64 PPP. This is 27th in the NBA among players who have defended at least 10 isolations...but 5th for players who have defended at least 30 isolations:
RJ Barrett, 30, 0.60
Kyle Anderson, 49, 0.61
Herb Jones, 32, 0.63
Kevin Durant, 61, 0.64
Keegan Murray, 75, 0.64
As you can see, he is the best in the NBA among players who have defended at least 75 isolations this year, and on top of that he draws the iso assignment, as only 9 players have defended more isolations - and all of them are bigs except Luka and Keegan, so most of those are probably picking up a guard who has penetrated to the paint, whereas Keegan typically picks up these assignments starting at the perimeter.
His FG% allowed in iso is vanishingly small - 24.6% - but his overall numbers only put him at that 92.6th percentile because he does give more than his share of fouls/FTs in those situations. He fouls in iso 14.7% of the time, while Rudy Gobert on similar attempts fouls only 3.8% of the time - I'll leave it up to the eye test to determine whether Keegan really fouls in iso defense 4x as frequently as Gobert.
Edit - I see Telemachus put up a tweet with similar data that is from pre All-Star break while I was writing. Keegan's numbers have slumped a bit, but still are top notch.