How to get the last date updated of postmeta?


WordPress does not track that information. You can’t find it because it does not exist.

Unless you specifically wrote code to track when post meta was added/modified, that information doesn’t exist, and it’s rare that any plugin would implement this as it would result in a lot of data collection.

It would also be obvious where that data was when looking at the database via PHPMyAdmin, which I and yourself did not see in the screenshot. So this data was never recorded, does not exist, and cannot be used. It would be a fools errand to continue the search.

The best hope you have is to check backups of your site and compare them by date to get a rough estimate of how far back the data existed. Emails/receipts/invoices might also corroborate this, but nothing in WordPress exists to tell you when a post meta was created or last modified.

The only other thing, is the post creation date, the post meta creation date must be the same or newer, but not older.