Hey @BarryK,
My name is Logan with Klaviyo's support team and I am happy to help out :) Our back-in-stock feature is currently triggered by the inventory level of your products across all inventory locations. Therefore, an item being in draft will not have an impact on flow triggering. I will let our product development team know that you would like this ability. Hopefully we can offer this through a future update.
If you want to ensure messages are not sent you can create a trigger split after the delay block in your flow. You can then have that split look for the product in question and have them queue up in a manual version of your message. Once you are ready to send those messages you can go into the flow and manually send them:
https://help.klaviyo.com/hc/en-us/articles/115002779331-Manual-Sending-and-Sending-to-Needs-Review-Recipients
Overall, we recommend not adding inventory to a product unless you want your flow to send messages. Thanks for your patience on this request.
~Logan