The 21:9 aspect ratio, called ultrawide, is basically a format larger than 16:9. In this format, the width has 5 more units, which makes the image longer, as in CinemaScope format for example. Currently several models of mobile devices use this ratio and you certainly can already see some videos on Youtube with this ratio.
WFHD · 2560 × 1080 21.34:9 a:r
WQHD · 3440 × 1440 21.53:9 a:r
WQHD+ · 3840 × 1600 21.6:9 a:r
WUHD · 5120 × 2160 21.34:9 a:r
5K UW · 5760 × 2400 21.6:9 a:r
7K UW · 7680 × 3200 21.6:9 a:r
8K UW · 8640 × 3600 21.6:9 a:r
10K UW · 10240 × 4320 21.334:9 a:r
21:9 is still underused on TVs, but has started to appear on monitors and cell phones. In the case of monitors for example, it started to become popular for use in games and video editions, as it has a broader view. In cell phones it also started to be used recently in high-end models, to have bigger screens for the most diverse uses.
However this format does not threaten the popular 16:9, as it is not any type of production that works well in the 21:9 format. You can already see videos on youtube in this format, but it is still rare.
Unlike using in digital media, you always can use for printing as it is quite common, especially for banners and advertisements. In the near future, we expect this proportion to be more used, mainly in TVs and cinemas. As this way we will have more immersion in the content offered.