This differs from my own experience with C++. I went from language-lawyer level knowledge (in some parts) while using it heavily (at work, at play and answering Stack Overflow questions on it) to forgetting quite basic stuff after a few years of no use at all. This was all before LLMs were on the scene. With that said, relearning is much faster than learning the first time, as the basic "shape" of things is still there in my mind.

In my defence, even Stroustrup acknowledges the language is too big and complicated, and it's continually getting bigger.