With the current staffing problems, I’ve had multiple experiences of flights boarding early and closing right on time (even sometimes on delayed flights) so boarding staff could move to the next flight, so I wouldn’t take the risk of arriving after t-20 these days unless delay is explicit. Boarding starts when it starts and ends as soon as possible, but staff don’t have time to linger around at the gate as they used to so to me this is not a risk worth taking.