The notebook entry continues to appear in OneNote for the web

A deleted notebook can leave behind an entry in OneNote’s recent list. That doesn’t necessarily mean the notebook still exists.

Check its original location in OneDrive or SharePoint. If it’s gone and the OneNote entry no longer opens, look for Remove from list or Hide in the entry’s menu, if available. You don’t need to permanently empty the Recycle Bin just to remove a stale listing.

Since you’ve already followed support’s steps, check whether the entry also appears in a private browser window. If it does, ask support to reopen the case as a stale notebook listing, including a screenshot and the notebook link shared privately. Repeating the deletion or clearing the same browser cache again is unlikely to address an account-side listing issue.