One thing that has been a staple since win95 days is the ability to resize windows, sidebars, columns; almost everything, really. Rarely is this found on the web today, or so poorly implemented that it’s thrashing the UI thread when one or more resize callbacks are triggered. Even without that, layout + painting alone would probably lose the race against 95 UI tech.
Another “legacy” UI that stood out to me is blender. It looks decades old (until it was revamped recently), is probably a hodgepodge of C/C++ but stays smooth as everything can be resized even on dated hardware.
I can’t help but think that the UI engineers of old were a much more professional bunch, and that the field has mostly regressed since then.