This is a bit odd, though, since general CT enforcement in Chrome didn’t start until Chrome 68. Also, Chrome automatically turns off CT enforcement if it hasn’t been updated for 18 weeks.
@afagund, do you know which exact sites have this problem? Chrome 66 did have CT enforcement for one CA: Symantec. If you have a site running an old Symantec certificate, it’s possible Chrome 66 could give that error.
At any rate, all this detail is mainly for curiosity and education. Your main message to affected users should be to keep their Chrome up to date. 