I do agree that WebGL 1.0 support is useful, though WebGL 2.0 support will surely ramp up given time. The main use case of the extension is VR, and especially on devices which are VR capable I'd expect widespread support for
WebGL 2.0 already.
Of the top 10 performing mobile devices OVR_multiview support is:
Nvidia Shield Android TV: NO
Google Pixel C (tegra): NO
Samsung Galaxy S8 (Mali-G71): yes
Nubia Z17 (Adreno 540): yes
ZenFone 4 Pro (Adreno 530): yes
OnePlus 5 (Adreno 540): yes
Moto Z2 (Adreno 540): yes
Sony Xperia XZ Premium (Adreno 540): yes
Essential PH-1 (Adreno 540): yes
Xiaomi MI 6 (Adreno 540): yes
I can't help but notice that the big gap in that picture is the fault of Google/Nvidia. It's a bit confusing since Nvidia was a major contributor to OVR_multivew spec along with Oculus, Qualcomm and Google.