Seems like a human solution to a (mostly) technical problem. Basically you just need a way for students to mark a message as hand-raising (and maybe a way to point out another student's message), a chat filter for the teacher to read those messages, and a USB LED (or some such) to act as a raised hand until the messages are marked as read. Of course there are some edge cases, like temporarily ignoring a child who's repeatedly raising their hand to troll, but this still seems like it could've easily been usable by the start of fall 2020.
You'd likely still need a surrogate for young children and special ed students, but you could probably eliminate ~2/3rds of them (? I'm not sure how old children need to be before they can be expected to remember they need to mark their messages & how to do so)