A quantum computer running Shor's algorithm could theoretically recover an incoming viewing key from a publicly exposed Zcash address to link an account's wider payment history.

From the articleZakura Proposes One-Time Viewing Keys to Stop Quantum Linking of Zcash SwapsAlso in Zcash developer proposes quantum privacy fix for NEAR Intents swaps, Zcash Developer Zakura Proposes Fix to Stop Quantum Computers Linking Shielded Swaps - Decrypt, ZEC tops $1,171 as quantum-proofing work speeds up - Inside Zakura's proposal - AMBCrypto, Zcash Proposes New Privacy Layer to Guard Swaps From Quantum Risks, Zcash Builder Zakura Proposes Per-Swap Viewing Keys to Close a Quantum Privacy Gap - Unchained, Zcash developers plan a fix to keep ZEC swaps private from future quantum computers