Disclosure: I'm in the UK where the worker protections are half-decent.

was it ever not OK?

It was not OK to not take a jobby on work time when you had the opportunity!

I've worked shifts where my relief staffer has been in twenty minutes early (long commute, unpredictable traffic) so I've handed the shift over, and ensured that the remainder of my time was spent losing half a kilo of weight in five minutes. Conversely, it feels far more productive to leave the house half-needing to give birth to a brown otter, and nip to the bog once your feet are clear of your workload that you've taken on from the previous shift to go and perform the bowel movement while being paid for it.

Shitting in my own khazi on my days off feels like voluntary work now.

Sensibly though, any manager who controls bog time is just a bit of a fanny. Unless someone is obviously taking the piss like spending four hours of a ten hour shift, then people will perform better once they've laid a cable whenever they've needed to.