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.
If they were popular enough (like all the examples you’ve named), they probably have been converted to a more modern format already (like all the examples you’ve named).
You could also download the files and still watch them on a local machine using flash player to enjoy them in the original format, assuming the .FLVs themselves are still available. Makes me wonder if Newgrounds still has any of that somewhere, even if not accessible by the public. They’re still around, but they’re modern videos now.