UPDATE: A NeoGAF mod has added this to the post for some clarity:
High performance events on Twitch require subs for HD, 480p replaced by 360p+
There are 2 types of subscriptions on Twitch.
Some streams (usually high-production tournament events) have required that you subscribe to their stream, to access 720p and up. In this scenario, the highest res you were able to get for free was 480p. Since there is now no longer a 480p transcode, the best you will be able to get is 360p. The vast majority of streams on Twitch on any given day are not these types of events with this subscription model.
The vast majority of streamer on Twitch are individuals. They often have subscriptions you can purchase too. However, from most cases I’ve seen, they don’t lock HD resolutions behind the subscription paywall, instead they tend to offer things like the ability to chat, or custom chat icons etc. In this situation, your resolution options will remain unchanged (unless you specifically liked watching at 480p).
Apparently Twitch are now calling users to pay a subscription fee for steams on graphics on 480p and above, Twitch support announced via twitter.
@DJHelium 480p was out all night carousing, actin a fool, so we had to put it in The Chokey. Medium is a high quality 360p now, think 360p+.
— Twitch Support (@TwitchTVSupport) August 27, 2013
It’s likely this won’t affect personal streams, but will affect bigger things like streamed MLG matches. This news comes just before the next generation consoles launch, which support Twitch. So really, I think we all predicted this.
Thanks, NeoGAF user DrKirby