It's currently the case that some purely WebGL 1 related extensions are exposed on WebGL 2 contexts which shouldn't be (because they're WebGL 2 core). It's also the case that it's not clear in the extension registry overview, which extensions are WebGL 1, 2 or both.
The extension definitions have the ability to mark up against which specification they're written, and they can also specify in which version they become core (example quoted from EXT_frag_depth below)
<depends> <api version="1.0"/> <core version="2.0"> <glsl version="300 es"/> </core> </depends>
Would it make sense to mark up/divide the extension in the overview in some fashion?