Obviously not the 'contiguous 48', but SFO-YYZ, which just started again in March, is currently redeye only on the easbound flight.
An asterisk that when UA has done this on-again, off-again route over the last, I don't know, 20 or 25 years?, it also had a daytime flight. At some point when it was supposed to start again, it was going to be 2x with a morning flight and redeye. The morning flight may be in the schedule for sometime in the future again, I'm not sure, or if so, when it might actually start.
I also suspect there are quite a few routes that are now 1x/day that are currently redeye only, but pre-pandemic ran with 2 or more frequencies including a redeye and daytime flight. I know SFO-IND used to have a daytime flight as well.