QB Issue Resolution:

It should show when you go on the sales page and at the top Click the outstanding ones. It’ll populate a list and you should be able to see the date that you sent the last reminder.

If you have a lot of outstanding invoices to manage might want to consider the app invoice Sherpa.  It handles outstanding  accounts receivable automatically for you.

Resolution for Issue 'Can I see the date I last sent an invoice reminder to a customer?' available: Yes (Solved).
Source: Intuit Community forum.

Can I see the date I last sent an invoice reminder to a customer?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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