@wojtek, I agree that rounded corners look a lot better in that example. However, that’s at quite a high resolution. On something like a 1080p display, there are benefits to the older design. There was reason for the current design once upon a time.
I presume I’m correct in estimating that the new icon theme shall take into consideration that the use case for the older icon design remains for the low-resolution hand-crafted bitmaps, whereas the higher-resolution SVGs shall be rounded.
@Enzo_Nunez, I agree. I once suggested that the shell use the server-side window decorations, but was informed that because this would break plasma themes, and consequently couldn’t be accepted. Perhaps Union shall remediate this.
If you’re using kwin-x11, you can use noborderrrule=2 in your .kwinrules.