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There are a couple of arguments to be made.
>Gen 5
First game to not feature then-staples like contests or secret bases. Removed following Pokemon from HGSS even though all 493 sprites were still in and functional.
>Gen 6
Removed difficulty modes from BW2 even though the game sorely needed it. X and Y are the only games in the series that don't make sense with their namesake without the addition of a third version, which in this case they never got and this could be considered the start of the trend of GameFreak starting new plot points without bringing them to a resolution. Also removed seasons to integrate them into the story which didn't end up having as good of an effect as they maybe wanted it to. Overall it continued a lot of the trends that Gen 5 started without any of the polish.
>Gen 7
Went as far as to remove series staples like the National Dex and Triples/Rotation. Removed a metric fuckton of QOL and improvements that Gen 6 made. Started the trend of exclusive gimmicks like Z-Moves. Started the trend of marker-based gameplay instead of any real exploration or substance. Brought back third versions in the worst way possible after Gen 6 forewent them and suffered for it. Started the trend of Kanto pandering to the nth degree.
>Gen 8
Continued many of the trends that Gen 7 started. Didn't fix the gameplay issues from the Alola games even though LGPE did. Didn't bring back any of the things that were removed from Gens 6, 5, or 4. First game to feature two separate subscription services to transfer Pokemon even though Bank was already promised to be the end-all-be-all solution. Excluded some Pokemon from being transferred despite this fact. First game to feature DLC, which, while not a bad thing, still didn't fix any of the issues that plagued the base game.