Maybe uMatrix is blocking them somehow? I've never seen an ad, even on channels I don't follow or subscribe to. My rulesets are very strict, but you're right, if they really come through the video stream I don't know how I'm blocking it.
Now, whenever I get an ad, I immediately close twitch and find something on YT/Mixer to watch. I'm training their algorithms to leave me alone.
I get ads when I use Twitch, with the same account, on a stock installation of Chrome (which I use as my backup browser for when I don’t want to figure out which extension is breaking a website). So it’s not related to my account, location, etc.