I am 30 and I find this hidden scrollbar fad (aka try-to-swipe-every-screen) really frustrating, am I already so old that UX designers should build special accessibility settings for me?

Sure, when I see younger people trying some new GUI they basically touch and swipe everything to discover hidden options and functionalities, because someone decided that a flat rectangle looked "cooler" than a button, or that a greyed out textbox looked more "modern" than a standard white one... but isn't this just bad design? what happened with make things as simple as possible, but not simpler and don't make me think?