While Flash enjoyed success from the late 1990s to the late 2000s (especially because of the failure of the Java platform in the web), it eventually declined as JavaScript and other browser-agnostic platforms like HTML5 and WebGL matured. Adobe, recognizing that Flash lost its relevance over time, announced in 2017 that in-browser Flash plugins would no longer be supported after December 2020; all major browsers permanently removed Flash support by the end of January 2021.
During its lifetime, Adobe made a change to the Flash player after version 9 that broke many older files. As of now, the 51 posts with this tag have been fixed by flipping a flag in the header, as detailed in topic #15331, but if such a file was found, it was recommended to try and view it in version 9's stand-alone player.
As of March 2021, Danbooru is set up to automatically load all Flash posts using Ruffle.js, which uses WebAssembly to emulate Flash in modern browsers as a part of the broader Ruffle Project. While this means you can view Flash posts without falling back on an outdated browser that's old enough to still support the original Flash Player, please keep in mind that Ruffle's core emulation of Flash is still in an alpha state and it's less likely to work with files that are newer and/or more complex. See ruffle compatible for posts that have been manually confirmed to work with the version of Ruffle.js that Danbooru is currently using.
