It's difficult to say without trying it. Some incompatible plugins work fine. Some won't load at all and become disabled (which stops all the functions using them from working). Some fail partially (E.g. one I wrote for 4.0 failed in 4.4 - it had some validators and post-functions, but the fail was limited to one post-function that just crashed on use)
You are going to struggle with this one. Finding out where you've used it means ploughing through all the places where you think you might have used it! If you have a small or medium Jira, you can export to xml and then do a search for the package name because the workflow definitions will mention them explicitly. For any Jira, a rummage through the database to at least find the workflows which are using them might be a good start.
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