Sadly the math I have forgotten the most is prepositional logic and proving things. It's probably the easiest to learn and pick up as well and is incredibly useful for reading proofs, but alas I never exercised that bit of my brain. Probably the math I do remember is the applicative bits of number theory and cryptography.
Having studied electrical engineering - communications in the early 1980s and then worked in software field ( enbededd software, large scale technical testing of remote systems ) for 30 plus years, I find recollection fades over the decades but it "comes back" at a reasonable speed.
SWE with a degree in Applied Math. It’s an easy decision to put me on the linear-algebra or Monte Carlo tickets. Only once has a specialist (in operations research) been called.