Is... That slang a play on the death stranding beach? Because if yes, it's kind of funny.

I learned the term at ThoughtWorks (consulting company), where it was a play on the phrase "on the bench", which itself meant "not working on a billable project". The "bench" version comes from sports like (American) football, where "on the bench" means "sidelined" or "out of action", which has a mildly negative connotation compared to the "beach", which sounds pleasant and fun.