Frequency illusion or Baader-Meinhof phenomenon is a cognitive bias where after learning something new for the first time you suddenly notice the same thing all the time. But there must be an opposite to this too.
I'm pretty sure there used to be things I came across frequently in the past. But then at some point they have disappeared. I don't notice those things anymore. I'm not sure if they actually even were a thing or did I just imagine it all. Maybe I just remember incorrectly.
Surely if we can ignore something as long as we haven't actively learned about it we can also fall back to that ignorance when we forget about it. It once again becomes something out of our limited focus while we find new things to focus on.
The weirdest thing is when you learn about the same thing again later. It feels familiar for some reason, but you can't quite identify why. It might even have a completely new name instead of the one you thought you used to know it.