You spend a non-trivial amount of time learning calculus, for example, but how do you maintain that competency?
If you can't remember the core intuitions about calculus (what does integrating a function mean, how to use the derivative or gradient to find local minima or maxima, which functions are continuous, which of those are derivable everywhere) then if you're like me and most people I know, you need to spend more time applying calculus.
You can read stuff in fields that benefit from application of calculus to get a better feel for it (geometry if you're doing anything with curves or curved surfaces; computational geometry is also quite fun, also in most engineering fields there is some calculus to model the application of the forces, if you like building things learning to simulate real-world structures and systems can be super motivating). Build a (very small) neural network without a framework, that'll make you work at grasping the concepts of measuring small variations in the output of a function in a way that is very goal-directed.
(I used your example of calculus because I think you're talking about being an adult with a day job who can't find a use for high school math. If you did math in university and are worried about losing your skill at writing proofs, you should find a job writing proofs. There is about zero overlap between proof writing and a developer career, and proof writing is tedious and difficult to gain the focus to do on the side.)