My remembrance of the specific quantum effects that you're thinking of are from quantum tunneling[1] of electrons. The problem occurs when the gate size gets small enough that electrons can pass through without the transistor being switched on, which starts to happen around 3nm.
https://en.wikipedia.org/wiki/3_nm_process gives a reasonable summary in its introduction.
My remembrance of the specific quantum effects that you're thinking of are from quantum tunneling[1] of electrons. The problem occurs when the gate size gets small enough that electrons can pass through without the transistor being switched on, which starts to happen around 3nm.
[1] https://en.wikipedia.org/wiki/Quantum_tunnelling