From my friends that work there - the answer is "it's complicated". Some of them do get away with it, obviously such as those that work on open-source software at Apple (like Safari, Darwin, etc) - or those with non-computer-related side-projects like photography or artisanal furniture making...

Apple seems to have inherited Microsoft's previous institutional paranoia about open-source software: the legal dept is concerned that if an employee casually browses GitHub and is inspired by some GPLv3 code that they could rewrite themselves for use in a proprietary Apple product then the lawyers consider that Apple product now possibly tainted (as rewriting code or being "inspired" by other code still legally counts as a derivative work, even if absolutely nothing was copy and pasted).

Microsoft lost that attitude around the same time Satya became CEO - I was at Microsoft when the transition happened and it was a refreshing change of policy that really came top-down. Of course we still had annual SBC training to remind us to always verify the licenses of any third-party or open-source components we use (and GPL is generally a big no-no, of course, without special dispensation from LCA) but the idea that a product's legal status could be tainted by casual browsing went by the wayside. I think a lot of the change came from a wave of sanity at the C-level when they realised the company was not actually being actively destroyed by meritorious - and meritless - open-source copyright violation lawsuits, and the few incidents that did occur (like the Windows ISO-to-USB tool debacle) were overblown with minimal impact to the company.

But Apple's paranoia isn't just about legal matters, but also out of concern that if Apple-watchers know who works for Apple and monitor their GitHub accounts then they'd be able to see which areas of technology interest those people, which may in-turn drop-hints about what Apple is working on (e.g. if we suddenly see a bunch of Apple folks Starring and forking LiDAR libraries for private use then that hints Apple is working on a LiDAR product... which they announced this week that they are: https://www.theverge.com/2020/3/18/21185959/ipad-pro-lidar-s...

Now, as someone who believes they'd otherwise make a great contribution to Apple as a valuable SE asset (yes, I'm being self-aggrandizing) this policy of theirs is a huge deal-killer for me. Not just because I own some popular GitHub repos with GPL licenses, but because I also have popular (and profitable) side-projects that only use a few hours of my time each month - and Apple is just as paranoid about those things as open-source projects are as vectors for leaking company movements, even unintentionally.

Heh - I remember shortly after I did NEO at MSFT and filled-out the "prior inventions" paperwork for my side-projects, my team lead admonished me for wasting his time looking at my dumb Google AdWords-supported websites - though he did agree the definition of "prior invention" was too vague.

(Footnote: if you're a recruiter or hiring-manager at Apple and you're reading this, and you agree that my side activities won't be a problem, please reply to this comment - I'd love to work on WebKit or Darwin at Apple :D!)