It depends on what parts we're looking at I guess, but I think flat looks good these days, I don't mind the rounded corners, I like the use of the palette in Win11 compared to Win10 (And definitely compared to Win98/2000 etc. which used too much contrast).
A lot of this is simply because the newer designs are based on large screens while Win98/2000 was hampered by the requirement to being readable at 800x600 and 256 colors.
Using gradients, negative space and was a luxury not afforded in 2000 but it does help.
In fact Windows 2000 use different color pallete than 9x as it does not try to fit into the 16 color RGBI CGA palette. The difference is subtle, but significant.
A lot of this is simply because the newer designs are based on large screens while Win98/2000 was hampered by the requirement to being readable at 800x600 and 256 colors. Using gradients, negative space and was a luxury not afforded in 2000 but it does help.