That doesn't explain why they would want to block IPv6 specifically, and not also block IPv4.

The OP's comment is that they can censor IPv4 when they want, but they don't know how to censor IPv6. So they block it entirely.

Thanks this really explains the situation.

A lot of the Starlink and other contraband uplinks are using ipv6, allowing connectivity for people the regime doesn't want to have contact with the rest of the world. They don't want the revolution broadcast or popularized.

I wouldn't think blocking terrestrial IPv6 links would have anything to do with blocking Starlink.

Since starlink supports v6, starlink users can p2p communicate with other v6 users. Both starlink and local carriers don't provide proper legacy connectivity, they are encumbered by cgnat so p2p does not work. Without p2p communications, users are forced into a client-server model and it's much easier to block a small number of servers rather than millions of potential peers.

It could be as simple as their surveillance / censorship tools not fully supporting IPv6.