Earlier quoted context omitted.
Can confirm from my experience (~10 years in the industry). I've never even heard of this practice until I applied to Google. None of the EU companies I've worked at before had anything like this. None of the EU companies I've worked at since had anything like this. It's US imperialism, nothing less.
That's nonsense. In Germany the default for contracts is that you need your employers permission to do side projects. Germany is in the EU.
> In Germany the default for contracts is that you need your employers permission to do side projects.
You generally need to notify your employer if you take on a second (paid) job. The employer may veto that employment, but must specify a good reason (you can't be in competition, exceed the total legal working hours, bring ill repute to your employer, ...), but the default is "allow". Clauses that require approval are generally not enforceable. As always, there are exceptions depending on the exact circumstances. See for example this discussion (in german) https://www.haufe.de/personal/arbeitsrecht/top-thema-nebenbe...
You generally don't need permission to do open source work, though the ownership of inventions that qualify under the ArbnErfG may fall to the employer. As such, it's good to get clarification and explicit approval from your employer beforehands.
(not legal advice, ...)