Hi,
I created Inventory History Guardian because of this post and the amount of people saying that it was an issue for them.
The app saves a record of the inventory of each product variant per location per day, allowing you to extend your inventory history beyond 90 days.
It also allows you to create inventory adjustments with custom notes, see all of them across your entire store in a single list view and again save them for more than 90 days whilst still working with Shopify’s inventory system.
Hopefully some of the posters in this thread will find the app useful. Let me know if you have any questions