Twitch and YouTube player, when not fullscreen, is 16:9.
Most users use 16:9 ratio monitors displays, but I am one of a few people to have old 1280x1024 monitor and an old CRT 4:3 ratio monitor capable of 1024x768 and 1600x1200.
When using fullscreen, it depends on viewer's monitor aspect ratio from resolution. Black bars is shown by video players to avoid distortion (skinny or fat objects).
Here are a few options.
1. Reduce monitor resolution to 1920x1080, found in windows display settings.
2. change base resolution to 1920x1080 or 2133x1200 and add some useful overlay stuff at one or both sides.
3. Continue to stream at 16:10 ratio (1920x1200 or 1280x800), as users that fullscreen to matching aspect ratio won't get black bars. Can go back to Twitch past recording to check this.
YouTube have some aspect ratio recommendation too.
https://support.google.com/youtube/answer/6375112