IME - Narrowbody flights can and do leave from any ATL concourse, regardless of destination. There isn't really any commitment to keep them in E/F if there's another gate that can handle the equipment. I see lots of Canada/Mexico/Caribbean flights especially departing from T/A/B/C/D all the time.

That said, a quick search on Flightaware shows the ATL-YYZ flights bouncing around between D and E concourses, for the most part.

Arriving back, you can also arrive into just about any concourse based on where the plane is going to next. Since YYZ flights are pre-cleared in YYZ, there is no requirement for them to arrive at E/F like other international flights (same for YUL, etc.).