Spieth is an interesting guy - hasn't won since 2017 and it feels like forever since he was dominant (2015). I think mainly its from losing his putting form. He was such an incredible putter then and to your point, was excellent at nailing the long putt. In 2018 he wasn't even top-100 in strokes gained from putting. Hard to win when the entire field is putting better than you.
He's only 25 but when I watch him now I feel like he lost his mental edge a bit and he legit feels this pressure.
Spieth, Justin Thomas and a couple of others from the "Bro Club" are going form being wunderkid prodigies surrounded by their families to moving on to having their own families, relationships and business stuff.
I get some good second/third hand info about players (A friend who worked with a player's father-in-law turned business mgr) that there is plenty of pull between their parents who helped forge their careers, and girlfriends/wives and sponsors, etc. Need to remember they are still kids (under age 25) and have a ton of money and access way before they're fully ready to handle it. No different than other sports. Most of these kids dropped out of college to start their golfing careers, and similar to pros in other sports, have so many people trying to get their claws in them.
You only need a bit of focus loss to go from major champion to run of the mill jobber on the tour. For Spieth I think he worried way to early about needing to get longer off the tee and his accuracy got destroyed. And, as others have noted in this thread, his putting has regressed to the norm.