The process pretty much started with Windows 8, which was released in 2012. I don't think the flat style will go away anytime soon. At most, skeuomorphic elements will be slowly phased in. Material design at least adds shadows, and "neumorphism" adds back some 3d popping out, although I haven't seen it much.
Windows 95 was probably the optimal UI—in the Windows world, at least. Meaningful buttons, a reasonably-contained set of components, proper scrollbars. I'm not sure what the next major revision was after that (98 didn't change that much, IIRC) but I bet it was probably a regression.
