The clauses I have seen attempt to claim all intellectual work regardless of when it was done or whether it was related to the job. To me, this is an indication that we truly have not progressed beyond wage slavery. I believe such clauses should be illegal. I think one thing we might be able to do is to create a list of companies that have blanket clauses like this and then some people (not most) will have the luxury…
Well there is one lady who got sued because she took her clients with her to a new firm. Didn't start a new company though.
But I have noticed that non-doers love to use this as an excuse to do nothing. In every happy hour/lunch, there is at least one person talking about a great idea but they not gonna do it because their evil employer made them sign bunch of documents. You tell these documents hold a little weight in court and they should go talk to a lawyer, they might agree but never will talk to a lawyer. Next happy hour they are complaining about same thing.
And if you tell them about your side business, they will do their best to scare you.
I work for major tech giant, and openly advertise my business on LinkedIn, resume, carry business cards. No issues so far. I don't talk about it at work or with non-doers.