Plug-in dependencies

I don't have a good solution to find current uses, but in the future a way to do this is to encapsulate your plug-in in an expression rule or process model, and then use that whenever you would need to use the plug-in. This gives you a few cool things:

1. You can easily find all uses of the plug-in by searching for dependents on the encapsulating expression rules / process models.
2. Now you only have one place to modify when you need to make changes to how the plug-in is called.

Also, that helps to replace the deprecated plugin with a new and similar plugin or when an equivalent OOTB feature is introduced.

For expressions, we unfotunately do not have a different solution from what you have mentioned.
For smart services, if you open the zip file created by Appian Health Check, there is a file called hc-process-model-nodes.csv which contains node types that you can filter by and their corresponding locations in process models.