I know that Av's and other security products have to clean out the deadwood so to speak. Removeing malwares that are no longer used much. I realize this is necessary to keep the program more manageable and efficient.
I sometimes wonder if after a time some of them are being reintroduced maybe even improved. One that comes to mind is a really old joke/nuisance virus/worm from long ago [think win 95 and 1.44 mb floppy].
It was not a real threat did not cause any system damage just infected what ever was used to type with or in.
Worked on all platforms/ browsers/ office/Open Office/Corel anything that you might write with filling forms on line or just typing a letter.
How it worked while typing words;lettere sometimes whole sentances would disappear or you would suddenly find your self typing in the middle of some line/sentence that had all ready been typed. Like I say just a nuisance no real harm.
Even back in the day many vendors did not consider it a high priority and did not even look for it.
However every version of windows sooner or later it's behavior would show up on every system I have ever owned or worked on.
I would assume that if some older virus showed up infecting enough systems that their definitions would be added back into the what to look for list of each vendor. However until then those who did get infected were just out of luck.
Just a real old timers observation. Just for reference for me this is happening once again at bleeping computer.