The problem is the countries, which censor Internet and block VPNs, also jam shortwave radio signals.

It's possible but also difficult to jam radio. That's part of why programs like Radio Free Asia[0,1] exist. Even if you can't broadcast from inside a territory you can broadcast from outside. It can be jammed but it is a tough cat and mouse game and jamming isn't precise. So when you jam there are causalities. Not to mention that jamming can be quite expensive.

I'm not saying that makes the problem easy, but I'll say that jamming isn't a very strong defense.

Though the bigger issue here is probably bandwith. It's hard to be both long range and data dense. There's probably easier ways to distribute this. Hell, both Koreas are known to transport different things via balloons.

[0] https://en.wikipedia.org/wiki/Radio_Free_Asia

[1] It is also why projects like Tor and Signal get funding from RFA. Maybe the US doesn't want encrypted services here, but if anything, it's for the same reason they do want encrypted services in other countries.

I’m not sure that’s super feasible any longer with the advent of cheap SDRs. Over-the-horizon HF broadcast can be heard with a simple speaker wire antenna inside your house. If anyone is interested in trying to deploy such an idea, I’d love to participate as an avid ham.

Could I ask for a source on that and how common it is?

Seems like it was used way back in the cold war (and even then not blocked/jammed) and I'd guess that current authoritarian regimes would perhaps not bother considering how few could use it.