Just as with muscles, skills wither when you don’t exercise these skills. Often when you are very skilled, the more you need this exercise or repetition to uphold this edge.
Most skills have a baseline where it’s difficult to dip under. As you might not be a pro cyclist any more without a training schedule, you will not forget to ride your bike. Of course, the more skilled you are, the higher this baseline usually becomes.
Some skills are obvious like Software engineering whilst others might actually be more difficult to notice, like Keeping things simple or learning.