Cut the shifts in half and people work less individually while more people overall have job opportunities. As a benefit, working shorter shifts would result in more rested workers who would theoretically be more productive or at least have less chance of mistakes.

This would either halve incomes or double labor costs. Any improvements in individual productivity would be offset by the new logistical nightmare (especially for knowledge workers) of coordinating with twice as many team members to accomplish the same amount of work. This seems like a recipe for skyrocketing costs.

Those workers may also be more likely to stay in work to finish a piece of work off, rather than leave as soon as the bell rings.

I humbly disagree. Even in medieval times there were clerics, scribes, book-keepers, and so on.

But those were the people that could actually read and write. Most people were illiterate. Only those privileged with status and wealth could have access to reading materials (and they would be the only people that would even desire them), and they were only men.

Those people probably made up less than 1% of the population.

The small number is irrelevant. The OP said office jobs did not exist in medieval times. This is quite clearly an incorrect statement. I am perfectly happy to debate whether pointing this out is relevant to the point being made, but I stand by my assertion.