From Homestar Runner to Salad fingers to badgers, stick figure battles, and the End of Ze World, this — dare I call it an artform? — was a cultural touchstone for a generation.
Flash made vector animation available to the masses, and internet distribution of the relatively small video files was a piece of cake. With the filetype now essentially deprecated, the creators gone on to bigger and better things, the distribution sites shut down, it is a dead form. Most of it will be lost forever, although there may be someone archiving some of it for posterity.
People jumped ship to prerendered videos even before the death or Flash, using Flash as the video player.
It’s been over a decade since I learned this, but if I recall correctly, SWF animations that were large enough had desync issues with the audio and frames. The solution was to export the animation as an actual video file and play that back.