"We do try to throw up appropriate error messages when a browser lacks the necessary support. (WebRTC isn’t required to use TogetherJS, only to use audio chat, so the error message there comes up if you try to activate that function without WebRTC.)"

So, the issue is not specifically WebRTC support here.

Edit: Also, the official browser support page: https://togetherjs.com/docs/#browser-support So, it could mostly work in IE10, but won't be a development focus.