In a decade of community management, I can count the number of times I’ve wanted to undo a warning on one hand. It’s extremely rare and the consequences of having stray warning are nearly imperceptible. When I haven’t had the option to reverse a warning, I’ve just left a reply along the lines of “Oops. My bad. Please ignore.” That way the user knows why they got the warning and future moderators can see it’s a false alarm. In addition, there’s usually something that caused me to notice this account and leaving the warning helps other mods (including future me) know what’s up.
I’m not saying this feature would have no value. But there are many other features that would be of greater value.