It depends on what you mean by "WebRTC". There's the mobile/native library (at webrtc.org) and the standard (at https://www.w3.org/TR/webrtc/). The library does support it (although you have to write your own signaling and media forwarding server). The standard does not (yet). But it's being worked on ine the W3C (for example, Insertable Media Processing like this: https://www.w3.org/2019/09/18-mediaprocessing-harald-inserta... or something more low-level like this: https://github.com/WICG/web-codecs/blob/master/explainer.md).

But the web standard not being there yet doesn't prevent someone from making a mobile or desktop app with e2ee conferencing, like Duo does: https://support.google.com/duo/answer/9280240?hl=en.

(I used to work at Google on WebRTC, Duo, Hangouts, and WebCodecs, but now work on video calling at Signal).