That depends very much on which jurisdiction you're in. You'd be crazy to assume, without taking proper legal advice, that retrospective assignment would not stand up as part of an explicit employment contract.

Like when I contribute to an already existing open source project on company's time/resource/IP-appropriation-regime

If you did that without your employer's permission, then probably both you personally and anyone redistributing the OSS project would be infringing the employer's copyright. If that upset your employer, you could expect to be fired and possibly sued, and anyone involved in redistributing the OSS project could expect a C&D from your former employer's lawyers pending removal of any infringing work you contributed.

Depending on the scale and sensitivity of whatever you contributed, a bad result could be that you are out of a job, bankrupt due to damages and legal costs, and known for being untrustworthy since any future employer who looks you up is going to find the lawsuit, while the OSS project is significantly inconvenienced and doesn't ultimately benefit from your contribution anyway.

and I do factor in the jurisdiction in my own personal assessments and everyone else should too

there are many circumstances where the consequences are completely opposite of what the contract says (or how their employer's actually operate, care about, or will ever find out about) and people's behavior will conform to those consequences

If it’s bad faith to sign off on a contract which contains clauses you do not intend to honour (because they are unenforceable) surely it’s equally bad faith (if not more so, given the power relationship) to keep unenforceable clauses in contracts purely in the hope or expectation that they will frighten uninformed employees into conforming with them?

or that my foresight and source of legal counsel is better and people may find value in that?

or I better hope my side project takes off right!