GoToMeeting claims end to end encryption[1] and in the same sentence say it's just SSL just like Zoom. Never the less they offer call-in as well so end to end becomes impossible right there. I have serious doubts about any conference software offering real end to end encryption as it's unrealistic for clients to be dealing with that many av streams.
End-to-end encryption doesn't require participants to receive full-quality video from everyone. Each client can be responsible for encoding their own video feed at multiple quality levels simultaneously – what WebRTC calls simulcasting. That does increase required processing power and upload bandwidth, but not to the point of infeasibility. And you do inevitably leak the identity of the person currently talking, as the server has to know whether to relay the high- or low-quality video stream for each participant to each other participant, and it can trivially tell the difference between the two based on bitrate. But that's much less bad than leaking the whole video stream.
>That does increase required processing power and upload bandwidth, but not to the point of infeasibility.
Most devices now do video encoding/decoding in a dedicated hardware unit, so those additional streams will have a vastly disproportionate impact on performance and power consumption. Some desktops and high-performance laptops support multiple streams, but most mobile devices don't. It's feasible, but very much non-trivial.
1. https://support.goto.com/meeting/help/security-faqs-g2m05001...