You can’t keep infinite knowledge in your brain. You forget skills you don’t use. Barring some pathology, if you’re doing something every day you won’t forget it.

If you’ve forgotten your Win32 reverse engineering skills I’m guessing you haven’t done much of that in a long time.

That said, it’s hard to truly forget something once you’ve learned it. If you had to start doing it again today, you’d learn it much faster this time than the first.