This is interesting, as I had an idea to do this when I was working on a competitor service during lockdown. Our idea was to use this to cut the cost on transcoded renditions, and for streamers that didn't have the ability to generate multiple renditions but still wanted them, we would offer it as a service in exchange for a higher cut of their revenue. That said, if anyone gets a benefit from lower quality renditions being distributed, it's Twitch.
If you think about it, the less bandwidth that Twitch consumes through their CDN network, the less money a stream costs per minute. A viewer still watches ads with the same CPM, a viewer still subscribes for the same amount per month per broadcaster, a viewer optionally pays the same for ad-free viewing (Turbo), and a viewer pays the same premium for bits, regardless of what quality they're watching at. So, interestingly enough, the economics of limiting transcoding are more about limited capacity and ensuring a positive cost per streamer. If a streamer does not become an affiliate or partnered broadcaster on the platform, the only revenue Twitch gets from the streamer is the ad money from its viewers. By virtue, they want as many casters on the platform as possible to become affiliates, as being able to get commissions from bit sales and subscriptions gives Twitch more revenue streams for the same content.
So, to your point, they definitely want to be able to support more efficient codecs that are not as patent encumbered as h.265 (which IIRC did have limited support in SE Asian markets), because at the end of the day, if streamers are generating the renditions for the ABR ladder, and ultimately, folks are using lower bitrates, then overall, this cuts costs for the same operational cost.
