As others pointed out, rows 20 and 21 have standard leg room (except 21A and 21F due to missing seats in 20A and 20F).
Row 21 doesn't exactly line up with the exit doors but it can be used to access the exit since 20A and 20F don't exist. So Delta designates all 3 rows as exit rows due to proximity to the exit doors.
Go with row 19 if you are going for the extra leg room.

What's really interesting is that I was in row 21 two weeks ago, and the FA didn't even have the willing-and-able-to-help conversation with our row.
Last week, I was in row 19, and that FA went over the exit row responsibility with all 3 rows.
Go figure.