With the death of Flash comes the loss of a huge part of internet cultural history, mostly interactive parts - as at least flash cartoons can be rendered to video and put up on YouTube. There were a lot of innovative games and concepts on sites like MiniClip and Newgrounds.

I remember seeing a very early demo of a <canvas>-based SWF player shortly after the iPhone came out - I wonder what would have happened to Flash if Adobe, Apple, or Google put effort into making it work the same way they did with the JavaScript-based PDF renderer.

Same with the death of Java applets. There were a lot of neat educational Java applets, especially in STEM subjects. Some have been recreated in JavaScript, but a lot are gone.

I remember downloading a VR web pages of a cell and atom in 2000s and hoping to get a magazine CD with a VRML player plugin. It may have been 25MB to download, but it took hours of billable phone time. Seems that dream is lost forever.