Adobe explains the trend over the past few decades as: helper apps became plugins, which in turn became open web standards.
Most of today's browsers ship with Flash pre-installed, but the plugin is disabled by default. Microsoft is also on board with this move and has announced plans to phase out Flash for Internet Explorer and Edge browsers by 2019, although end users will still have to option to re-enable Flash. This means that the Flash Player will keep getting regular security patches, get new features or capabilities, etc. Those companies have already started the transition away from the technology; their blog posts about the transition are linked above. Since Flash was pretty much the first tech of its type that was integrated into browsers, developer and user familiarity-coupled with the slow development and rollout of better open standards-ensured that it remained in use for far longer than it should ever had. Safari and Mozilla users will have to install the flash separately and will have to choose which website can access flash. As a result, we've partnered with game developers to support their HTML5 efforts, helping them continue to deliver games on Facebook.
When Adobe conceded a couple years ago that HTML5 was the new standard, it was only a matter of time before an official end of life announcement was made. However, that's a good thing, as the companies will have enough time to migrate to the HTML5.
Today, Adobe announced that Flash will no longer be supported after 2020.
Google is slowly phasing out the Flash-based content to HTML5 on the Google Chrome. Among many web analysts, Flash has been a punching bag for years because of concerns over its security abilities. "They're also more secure". Developed by Adobe, Flash had been sort of a requirement for years for anyone who wants to watch videos or play certain games on the internet. Google keeps an updated version of Flash inside Chrome, but it says that will end in 2020.
Apple was one of Flash's most vocal critics. The end of support in 2020 by all companies is a warning to the creators to move to the new standards until then.
Flash does not support touch. Most modern web browsers already make it fairly hard to run Flash content on websites, but Flash will be completely removed in web browsers by 2020. Meanwhile, Flash was riddled with security concerns and became a frequent target for hackers looking for malware delivery mechanisms.