It often strikes me that nothing has aged as well as solid colour backgrounds/widgets when it comes to user interfaces. If you'd told me this site was made in 2021 I'd believe you. Another example that comes to mind is the intro screen at the start of The Running Man, crisp white text on a red background. Looks as modern as the day it was made: https://i.imgur.com/wCRSDK0.jpeg

I wish I could extrapolate from this to figure out what else will age well when designing UIs.