I've seen the Mask Up sign on one segment. I want to say it was on SEA-DTW. I'm flying every week right now and I've found for the most part people are complying. I will say, the FA's seem less strict than they did when I had to travel back in March.
Last week, DTW-DCA, the pilot came out before leaving the gate and made an announcement telling people to comply and keep your mask on. Basically heading off any issues people would give to the FA's.
I am also fully vaccinated. Yes, the mask is a pain on long flights. With that said, there is no easy way for DL, or any airline, to track who has/has not been vaccinated. Instead it's easier to ask everyone to mask up. I think this is why the CDC has been hesitate to say vaccinated people don't need a mask. Then every store, restaurant, etc would have to figure this out. And there will be plenty of people who lie and say they're vaccinated just so they don't have to wear a mask.
Having been on a dozen flights in the past four weeks, and flying again this week, I can say the flight crews are thrilled to see full planes once again.