QB Issue Resolution:

To find inactive customer customers, go the blue navigation bar on the left

Select Customers (Customer Center)
Click small “Gear” on the far-middle-right (located next to excel link and print link)
Select “Include Active” under Other
Page will refresh itself, it will also include inactive customers labeled as deleted.

Resolution for Issue 'how do I find my inactive customers on this new version?' available: Yes (Solved).
Source: Intuit Community forum.

how do I find my inactive customers on this new version?: 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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