At least as a practical matter. Some large companies (in particular) may have draconian policies about doing moonlighting but it's probably not a big deal generally as long as it's low-visibility.
I avoid obvious conflicts of interest. And for any unpaid and pizza-paid advice/minor work for people I do it quietly.
But that stuff is pretty simple relative to a (minor) real side-gig which I had (shareware back in the day) but was pretty much orthogonal to my day job and my manager knew about.