It's simple - the last time a plugin was "used" was the last time JIRA was running.
The question doesn't actually make any sense for many add-ons. If it's providing a field for example, then as long as there's data for one or more issues, it's in use.
There are some add-ons for which you could say things like "this report/export has not been used by a user for X", but there's no way to track that in JIRA. There's no "history of usage" for any add-ons, even the ones where it could be done (which isn't that many)
I assume you are trying to reduce your add-on usage (and hence bill). The only way to do this is evaluate what you installed an add-on for and ask your users if they still need it. The harshest approach (I'd only do this after asking the users and having no-one admit to using something) is to simply disable an add-on and see if anyone yells at you.
I have multiple projects that use variations of the same base workflow. The variations depend on the requirements of the project or issue type. The variations mostly come in the form of new statuses ...
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