Perhaps something here will help you:
iPadOS - Safari - Lost Windows, Lost Tabs… - Apple Community
This article was originally written for iPadOS13 - and updated for iPadOS14 - but has not been updated to cover some minor differences that have appeared in iPadOS15. With iPadOS16 around the corner, I can’t see much point in updating the article until I’ve established and understand the substantial changes that are anticipated.
A principal change within iPadOS15 is the method by which you can recover an accidentally closed Safari instance. For iPadOS15 - starting from a Home Screen with Safari visible in your iPad dock - touch-and-hold the Safari icon in the Dock; an action menu will appear.
From the menu, tap Show All Windows; a screen that appears very similar to the App Switcher will appear showing all of your currently open Safari instances. At top right of the screen, next to the “+” button, you may see a button with which to re-open recently closed Safari windows. Re-opening the recently closed windows will open the Safari instances with all of the associated tabs and tab history.
While I’ve not established how long it is possible to recover a closed Safari window, the period is finite.