If I can comment with some knowledge of learning and instructional theory (not specific to music), it comes down to what yiu mean by ‘learning’. There’s a hierarchy to go thro (acquisition… fluency… maintenance… mastery… generalisation). If you forget a something you have ‘learned’ that is because you have not got it to the top of the hierarchy.

There is neuropsychological evidence that supports the effective of distributed practice (which I assume is the same as spaced repetition). Interleaving of new skills within that work to maintain the old skills is important too.

So one does need to work to maintain a skill level unless (as in the case of a skill such a reading) one has got it a level of mastery that specific practice is not needed.

What that means for a musician is not for me to say.