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.