Additional team and channel-related features and issues within Microsoft Teams for business
Your observation is correct. When you add a Whiteboard as a tab in a Teams channel, the actual .whiteboard file is stored in the OneDrive for Business of the user who created it, rather than in the Team's SharePoint site. The Teams channel provides access to the board but does not change its storage location or ownership. Therefore, if your account is deleted when you leave the institution, you should not rely on the Whiteboard remaining available indefinitely to your coworkers.
After the account is deleted, the user's OneDrive is retained according to the organization's OneDrive retention settings. During that retention period, users who already have access may still be able to access the Whiteboard. To avoid depending on the retention period, I would recommend asking your Microsoft 365 administrator to handle the Whiteboard before your account is deleted. Microsoft documents a process for administrators managing Whiteboard data when a user leaves an organization, including accessing the former user's OneDrive and moving the .whiteboard file to another active user's OneDrive.
If you're unsure who your IT administrator is, please refer to How do I find my Microsoft 365 admin? - Microsoft Support
You can read more at Manage data for Microsoft Whiteboard.
I hope this information is helpful.