If you weren’t paying attention to a thing, you won’t remember the thing.
In order for new dendrites to form and for you to learn, you have to actually be aware of the thing. We don’t know if the brain prunes what to pay attention to after sensory input, or if it’s somehow pre-screened by the sensory system.
There are forgetting neurons. They were discovered in 2019. They help us prune what’s not important. Keeping what’s essential.