2019-20 was another off year, and the 3 year period is one where his 3pt shooting tailed off relative to the league, his defense tailed off, and in 2020 his 3PAr cratered compared to the league. 2020-21 was better than 2019-20, but the whole league was way up (57% TS), and he rode the wave. It's not a significant trail off, but his last 3 years show some concerning signs. Maybe it's nothing and he turns it around, but I was comparing him with Dame who despite being 3 years older and who had a similarly injury filled year has been the model of consistency the last 3 years.
The Celtics will almost certainly not S&T for Beal, it would hard cap them and given they are already at the hard cap line, that would mean sending out close to $45M in salary for Beal. The only (unlikely) situation in which the Celtics could get Beal this summer is if he opts in to his deal then we trade something like Horford and Theis for him.