Keep in mind you cannot force a 21:9 source into a 16:9 destination / vice versa. Different aspect ratio means there is some part of the picture in one aspect ratio that is missing in the other aspect ratio. In 21:9 there is more picture to the right and to the left, and missing picture info on the top and the bottom than in 16:9, so if you don't want to cut that additional picture info, you need to add black bars to the top and to the bottom to keep aspect ratio of the actual picture info.
The other way round is if you have 16:9 material and want to show it on a 21:9 monitor: in the 16:9 material, there is more picture info to the top and to the bottom, and missing picture on the right and the left, so a non distorting conversion has to add black bars to the left and to the right.
So if you intend to produce content for viewers other than you personally, you need to look what aspect ratio their media players will have. The vast majority of users has 16:9 aspect ratio monitors, so they will have 16:9 players. See the
Steam hardware survey. To provide these users with the best video, you need to produce the same aspect ratio, i. e. 16:9.
If you producing game videos, this means if you don't want to show black bars or distort the video by stretching it, you need to play and record the video with a 16:9 resolution, even if you have a nice widescreen monitor with 21:9 aspect ratio. Set your game to 1920x1080, set OBS canvas resolution to 1920x1080 and record this. Your game will have black bars to the left and to the right of your monitor, but the footage you create will have no black bars on the monitors of your viewers.
