QB Issue Resolution:

You can try to delete it, but something In Use won’t delete.

This is Basic List Management: Open the Lists menu > Templates List. Right click that one and make it Inactive. Now it never shows in the Dropdowns.

That is true for Templates, Names, Accounts, Class, Items, etc.

Resolution for Issue 'IF I DELETE A TEMPLATE WHAT HAPPENS TO EXISTING ESTIMATES that used it' available: Yes (Solved).
Source: Intuit Community forum.

IF I DELETE A TEMPLATE WHAT HAPPENS TO EXISTING ESTIMATES that used it: 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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