Baseball has changed a lot since the 1990s. The players a big part of that change because they have different playstyles, different strategies, and home run or strikeout celebrations have became a big thing. The players in the 1990s weren't as sensitive as the players today. The relief pitchers in the 1990s used to pitch three innings at times but now you see no relief pitchers going more than two innings.
Another thing that has changed in the MLB since the 1990s is the players playstyles, like mentioned above. They seem to bunt less and try hitting more home runs than for contact. Pitchers now try to throw as hard as they can to get hitters out. The managers try different playstyles with their relievers and starters.
Ever since the 1990s, there has been rulechanges. Back in the 1990s, catchers were allowed to block the plate. In 2014, they made multiple rule changes. One of them just mentioned was catchers not being allowed to block the plate. Another big thing they added to the MLB in 2014 was challenges where you can challenge an on field call. Lastly, in 2023, they added a pitch clock which is where the pitchers have to throw the pitch before the timer runs out and they added bigger bases.
The last big thing from the 1990s til now is MLB umpires. MLB umpires were different from the 1990s where now they track umpires by their accuracy on balls and strikes calls. Another big difference is the umpires nowadays are less likely to agrue as much with the players. The biggest difference is that within a few years, MLB has said that they may add robot umpires because of the peoples dismay of the ball and strike calls.