This may seem a software question but it's at the OS level and all the few linux distributions I've tried don't include gradients, so it seemed more appropriate to me for it to be in this forum. As example, Android, MacOS and Windows have icons and decorations that use color gradients for creating a more three dimensional look. Is there a linux distribution?

I guess that does not depend on the distribution and/or OS, but something else. But I don't really understand what are you speaking about. You can define any kind of icons for your desktop. Can you show a few examples about that missing color gradients?

I believe what you speak of is the fashion/style of using flat UI themes - back when you started using Linux the styles definitely included more gradients.
It's just a style though, there's no technical reasons. Find themes and icons with gradients, you have gradients. Proof:

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You are right, that does not depend on the distribution and/or OS. But the OS or distribution may or will contain them. They are just gradual changes in color as in photograph. When color gradients are used in icons, buttons and decorations they give the illusion of depth and add visual interest. But they also tend to clutter the desktop. My curiosity was satisfied by ondoho's information above that states that it is the current style to use flat UI themes and implies that it has been so for a while. Thank you for your comments.