Does pruning old block data mean Kaspa's history can be altered?

No — pruning old block data does not allow anyone to alter Kaspa's transaction history. Even though Kaspa removes old block data to keep node storage manageable, the ordering of every historical transaction is permanently locked into every new block via KIP-15's recursive cryptographic hash chain. Think of it like a wax seal on every block: the seal encodes the entire chain of prior orderings, so if anyone tried to change even a single past transaction, the seal on every subsequent block would break. Any node on the network would immediately detect the tampering, making silent history rewrites impossible.