This is quite the recent removal then. I recall browsing a US government website around the start of the covid pandemic where one could check your entry and exit record.
They do keep track of when you leave by commercial air or sea, and I think there are data sharing partnerships with Canada/Mexico as well. Not sure if they keep track for citizens though, since there's no immigration enforcement reason for that.
They're working on an "entry data is automatically sent to other country as exit data" system with Canada. I'm not sure they've got any decent partnership with Mexico currently, though I know they've been negotiating.