As much as every other platform not just Twitch the lack of information for those who stream at other aspect ratios rather than 16:9 is lacking to non-existing.

It would be nice to have more support for those content creators who do not follow the standard.

The best example would be in my case is when streaming at 1080p my stream width is 2560 and not 1920.
This effect the stream in ways that sometimes it shows and sometimes it's just a black screen (Reported by viewers)

Better support for other aspect ratios rather than 16:9
Better docs to support a variety of aspect ratios (At least the most common ones 4:3 | 16:9 | 21:9 | 32:9) Which will include the required bit rate and stream settings. For example, for the best setting for ultrawide 21:9, I found that 2560x1080 resolution is problematic altho better performance can be found when streaming at 2292x960.

Twitch step up :)