Where are my "exchange" reminders in mac OS?
When the account is removed on iOS or macOS - the data is not deleted and only the local cache of it goes away. This is true for any sort of exchange or iCloud or google data lives in the cloud. Thus, the best way to find missing items would be to open the web client and look at the data store directly.

Once you verify that the data exists in the cloud - go ahead and add back that account on your Mac and enable the data classes you want (tasks map to reminders).