How can you be sure you remember if you aren’t actually doing it?

This is an important question I think. Gradually losing a skill to atrophy is not something you notice consciously.

Come on, I've been coding for 45 years. I don't forget so quickly.

I am sure you can still code, but there is no question that there are certain small bits that you no longer remember as well (or at all) as just 6 months ago.

I believe - but cannot prove - that the atrophy follows an S curve (decreasing with time), so that in the beginning not much happens but with time the rate of forgetting things increases.

There's a difference between forgetting the minutiae and still having the skills.

One doesn't forget how to learn. It's just that you've switched to learning something else.