Most of the most impactful strikes in history have been illegal. It’s easier if they’re legal but it’s not a requirement.
I feel like it wouldn’t bad too bad an idea if some kind of important stuff was illegal at all times, just to keep people in the right mind space. I actually used to have a whole conspiracy theory that that was why drugs were illegal in the United States, so that it would drive a whole shadow economy and battle-tested organization that could keep getting away with supplying them and wouldn’t get soft and complacent, so the US would stay sharp and have a wellspring of behavioral wilderness to draw from so we could stay competitive in the cut and thrust of the world stage. I don’t think that anymore. But I think it’s good to get used to fighting for your stuff.