There is a trove of flash animation that is completely lost to history
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.
They're not lost, most of them are archived via Flashpoint. The most notable ones have also been exported as regular videos on sites like Newgrounds. But yeah, I miss that Flash era where people made fun animations and games for whatever was on their mind.
The thing I find that is lost is the blurring of the line between video game and animation. Homestar Runner cartoons were often interactive, they made several outright games but also the things that were closer to animations often had easter eggs in them, from (in Strong Bad's words) dumb stuff that would pop up to entire extra scenes.
Early Youtube had a thriving animation community, but given the limitations of video-based content they really couldn't do those interactive elements, then Flash died, and now that culture is basically gone.