A customer of ours suggested this feature;

It should be possible to have the option to add deactivated products to a manually placed order. Perhaps with a checkbox while we're adding the products or clearly mark the deactivated products in order to avoid mixing them up with active ones, in case product names are rather similar.

The specific usecase is this:

That merchant frequently places manual orders in their Admin for customers. Contrary to what one might assume (I did so as well), it is not a once-in-a-lifetime event that while you work on said manual order, another customer in the frontend already buys the stock away, leading to the product no longer being available. The shop owner reported THREE separate occasions where this happened and thus, understandably, now seeks a solution for these cases.

There are workarounds of course, but they all come with advantages and disadvantages and at least I was not able to think of any which is really failure proof. They all also require multiple steps to get them working (like manually adjusting stock amounts before the manual order and then after again) other than automating everything via API, which sort of defeats the point of Admin orders.

Assuming we tackle this problem, it should most definitely be a setting or option because if we show any and all products, even deactivated ones, as possible selection during the creation of manual orders, we'll probably introduce new problems for shops which only have products available once and never again afterwards. In these cases, the products likely end up disabled once they're sold out and they'd NOT want those to clutter the dropdown list when placing an Admin order.