Find Variable or Text
There are some other great suggestions for documentation of variables, but I would also like to see a way in the designer to highlight actions that use a variable. I envision a Search button in the ribbon that opens a search dialog where I could either select a variable from the list of variables in this workflow, or simply type something in a text box. The feature would search the workflow and highlight all the actions that use the variable or contain the text.
The suggestion for finding and removing unused variables (https://nintex.uservoice.com/forums/229405-1-nintex-workflow-for-sharepoint/suggestions/10843149-find-and-remove-unused-variables) identifies the scenario of trying to delete a variable and getting the error message that it is in use. But where? This suggested Search feature would be a way to find that one action that shouldn't be using this variable so that it can be fixed and then the variable is unused and can be deleted.

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Drew Johnston commented
Agreed - it's frustrating to try to find where variables are used in your workflow.