In certain key areas Messi is half the player he was at his peak. Let's not fool ourselves here. Just because he gets paid more than ever doesn't mean he didn't obviously decline.
Totally. I think technically he improved a lot during his career, increasing his range of passes and quality of his finishing. His ball control still is amazing, controlling the ball in tight spaces like no one else. Mentally, he sure improved in his extraordinary vision, decision making and choosing the best possible moments to make a pass, a dribble or a finish. Still talking about mentality, he decreased in work rate, off the ball movement and personally, I think he lacks a good amount of motivation. I said before, nowadays Messi only gets fired up if someone hits him, a hard tackle, an elbow in his face. In these moments you can see and feel that he's going 100%.
Physically, he decreased a lot. It was one of his best parts, that mesmerizing combo: acceleration, agility, dribble and incredible change of direction. No one could deal with that in the past and I doubt 2018 defenders could do better against him. He still has the dribble part, the close control, but his body doesn't act like he used to do. If the defender is coming against him, he can dribble all day, just using body feints. His issue is when facing a standing defender and Messi and the defender are at 0 speed. His acceleration to beat his marker is not that fast anymore. For me, that's why he's having some much trouble in games with less space. He doesn't have the acceleration to dribble through the defender, so he has to make a simple pass, or pass and run to receive after a little sprint. A younger Messi would dribble the defender, and with that outstanding speed and free to go, he could choose to dribble, to pass, pretty much anything.
Of course it depends on his teammates, team formation, strategy, coach approach to the game and much more, but we can't deny his physical decline and how it's affecting his game. With teams giving less and less space, being fast and reacting quickly are important, and Messi surely decreased in both aspects.
Off topic: In Football Manager I tried him as a Poacher, doing nothing when defending, and when Barca were attacking he would wait glued to the defender and then make a run behind to receive a long pass. Worked really well because he was smart enough to not be offside and run at the right moment. Not sure if would work in real life.¨
Edit: I forgot to mention one thing that Messi surely decreased was his choice of friends.
Forget Suarez and let him go. Make new friendships with Malcom, Dembele, Coutinho, Arthur.