So I've worked in the game industry where this happens even more aggressively than the rest of tech, and I've had contracts in at least 2 jobs where I was explicitly forbidden from engaging in any activity outside of work (I don't remember how it was worded, but it was so strict that even doing charity work could have fallen under it).

That combined with the frequent contracts of "we own everything you make while you're employed by us" effectively means side projects are either forbidden, or can be stolen by the company at any time, which is similar.

Not all companies are that bad but it's still a thing, and a lot of employment contracts make it very hard to work on a side project without your company potentially being able to claim ownership of it.