I'm here to help you today so you'll be able to know what transactions have been matched by the specific user.
Do you want to run a report that lists all matched transactions by user? If so, as of now, there isn't an available report showing that information in QuickBooks Online (QBO).
However, you can open the Audit Log to check and see the modified transactions and matched transactions with the date of the event, and the user who made the changes.
To open the Audit Log:
For future reference, you may read this article to learn more about adding and matching downloaded banking transactions in QBO: Add and match downloaded banking transactions.
Let me know if you're referring to something else and if you have any other QuickBooks questions, I'll be always here to help you!