It depends on the nature of the side project (vs. side hustle), its relationship to your day job, and your employer's rules (and attitude). In my case I've done a number of tech-related books. My employer knew about (and encouraged) them. And we've had a tacit understanding that I wouldn't be doing these purely on my own time and I'd continue to work on my "day job." It was never a problem and worked out for both of us.

I certainly know people who do similar things around open source projects.