In P1.0 of this book, in the "Wiring up the A/V Toggle Logic in JavaScript, it says:

“The toggleVideo() function is almost identical to toggleMic(), but with one key difference: by disabling a video track (video.enabled = false), the video will appear to be frozen when in reality the peer will stop sending new frames of video. But a frozen video frame will make a live stream appear broken. So we’ll need an additional piece of logic to add or remove the video track on $self.mediaStream:”

But after testing, it seems that setting video.enabled = false does keep sending frames, just black ones (also mentioned here: MediaStreamTrack: enabled property - Web APIs | MDN). What causes the self video to be frozen is actually using the removeTrack in the same example - that will stop showing video frames at all in the self video.