Do I need a BoB when replacing a parallel port connection with a UC300 or UC400?

[A_Camera]

The (stepper) driver normally provides the optical isolation, rather than the BOB but the BOB typically provides the optical isolation for the sensor inputs. All good stuff.

Some BOBs do provide optically isolated outputs to the (stepper) drivers, absolutely. But I’d suggest 95% (random number, guess) of the enthusiast grade machines will have optical isolation within the driver, and adding it also to the BOB is not required (and can be detrimental)

Be aware that many drivers also implement optical isolation the wrong way, so in reality, even if there are opto couplers on the driver PCB and they claim optical isolation, it does not mean that it provides protection when needed. I tried to google documentation about the driver the Op mentioned but I failed, so I have no idea how it is actually built.