Most of the large diagrams I make use fewer than 2^8 colors, but then I tend to use solid-color stuff rather than gradients. Granted, SVG is even better for those, but if they need to be rasterized (as they often do), PNG8 works fine.
I like to use slight gradients and/or antialiased text. Both use up palette entries like crazy.
Good point about SVG(.gz), I forgot about it. SVG it can do transparency as well, and is extremely compact, at least for diagrams. And even better, it allows to add interactivity to your diagrams.
WEBP would be as unsuited as JPEG for diagrams, as your high frequency drawing will be turned into a blurry mess. However, for things like layer effects and backgrounds, it'd still be nice to have a lossy raster format that supports partial transparency.
