They are (were) maintained by a single entity that seeded all updates and fixes.
Plugins, like Flash, the long-dead Silverlight, and the infamous Java browser plugin, operate under a closed source development model. Anyone can look at source code and probe for exploits or implement the technology in their own projects. Open source technology is often held to a higher level of scrutiny. Unlike Flash, these open technologies don't require a third-party plugin.
The web has been moving away from Flash for years as browser-based technologies, like HTML5, WebGL, and WebAssembly, become more widespread. Adobe announced that it will 'stop updating and distributing the Flash Player at the end of 2020.' The company encouraged content creators to migrate any existing Flash content to 'new and open formats.'
