You might like this: https://nielssp.dk/guis/buttons

I see a couple that look like buttons to me, like a yellow Donate button and a green Learn More button.

I agree that most of the other ones don’t look very button like to me either. But even so I would probably understand that most of them were buttons when I saw them in context on a page.

The flat style has been around for a while now, so I’ve gotten used to it even though when I started using computers it was on Windows 95 with the grey rectangular buttons with shaded sides that give them the “3d effect”.

And even flat style buttons often have changes when you hover and click, but it seems that they didn’t copy that part on this button collection page.

It sounds like you're talking about the "gloss" era with "juicy" buttons, which some people are retroactively referring to as Frutiger Aero. Yes, things definitely look more 3D in that design. It's currently considered out of mainstream fashion, which explains the lack. Will it resurge?