An average user of our online store places an order every week. Is it reasonable to drop items out of the order history after 6 months or should these transactions always be accessible from their online account?
Any cut-off you make will be at an arbitrary point in time, which may make sense for some users- but by equal amount not make sense for those who require access to legacy information.
If you have the capacity to retain and provide access to all records, absolutely do so. Implementing arbitrary cut-offs break usage logic for the most part.
With this in mind, you could however, provide full fidelity access to recent orders (last 10, 6 months etc) and then provide lower fidelity access to previous orders before this point (simply item number, name, quantity, ref, cost, date etc). Thereby reaching a compromise.
As you state, the business asked you only show the last 6 months of orders. You could simply make this the default but then provide access on request to further information ('request' could simply be a mouse click).