I'm looking for a report(/saved search?) to show me all items we had in stock as of 12/31/2020. Does this exist natively?

Reports > Inventory/Items > Inventory Valuation Summary. You can set the "as of" date. It is sourced by summarizing transaction history. You'll see this if you dive down into the detail. Alternatively, one of the other native inventory reports might also work for you (e.g., Stock Ledger, etc.).

Perfect- thanks

It should be noted - any backdated transactions could goof the accuracy of it, and make it inaccurate from what was physically in stock - or what was in the system as of the as of date.