The scrollbar is really hard to see - especially on wide views like Planner. In a way I could see where you were going in making it less obtrusive, but it becomes harder to find.
Scroll bars are not visible unless user mouses over - this is a more general Teams problem.
We have a horizontal scroll bar that is too thin to click. In some Microsoft products this new design makes the scrollbar thicker when the mouse hovers on it. This scrollbar doesn't. They need to be revised.
Scrollbar is awful. When using it in a busy channel it jumps around too much when controlling with mouse drag. Should use standard control so individual clicks can be used to step gradually through the content.
Sean Ellis commented
Classic example of inconsistent UI. The user spends the majority of their time in other applications which have the OS default scrollbar behavior. Teams should follow Microsoft's own UI guidelines and use common controls.