SWFUpload is an open-source library that was used by WordPress in 2011 and earlier that allowed users to upload files. The library was abandoned and replaced with Plupload in WordPress 3.3, released in 2011. Despite being replaced, WordPress continued to bundle the library for plugins that didn’t migrate to Plupload. In 2013, the core team forked SWFUpload and maintained the project, applying security fixes submitted by contributors.
After six years of deprecation, the core team has announced that SWFUpload will officially be removed from WordPress core. The team searched the WordPress plugin directory and compiled a list of plugins that contain references to swfupload in their code. According to Weston Ruter, the list includes 128 themes and plugins.
Some of the most popular plugins include:
The team is working on a way to provide enough backwards compatibility to ensure there are no JavaScript errors and an upload form is displayed instead of embedded Flash. Andrew Ozz also apologized to plugin authors noting that the list likely contains some false positives. If you use one or more of the plugins mentioned above, please get in touch with the author and ask if it will function without SWFUpload in core.
Nextgen Gallery is also on that list; with +1mio active installs it seems to be the “biggest” one on the list and one can only wonder why you don’t mention it in your article?