2000kbps is the advised real-world maximum recommended for non-partners, to minimize buffering issues and remain most widely watchable.
The 3500kbps bitrate is the maximum the ingest servers are vetted to handle correctly.
Going past 3500 is not advised, and is "here there be dragons" territory; if your stream starts to screw up, Twitch won't be able to help you until you drop the bitrate to that safe ingest max. So even if you can do it, it's out-of-spec and may screw up badly. It's like one of those yellow speed-advisory street signs on curves. They don't actually change the speed limit, but they're the estimated safe maximum.
Staff have said that the point where you may end up banned as a denial-of-service attack starts around 6000kbps, but it's more or less a danger zone to avoid (highest I ever run is 5mbps, and only for a few specific games that truly deserve it). Past that should be discussed and pre-cleared with Partner Help unless you have at least a four-digit concurrent viewer average.
I am not Twitch Staff. This information is taken from discussions I have had with staffers, posts they've made over the years, but should not be taken as any kind of official communication. As with everything on Twitch, the above may change because it's Tuesday, or someone saw a cat outside their office window.