ParameterList was marked as deprecated as a reminder not be used in any *new* APIs or custom code. This is to make things slightly easier when it is eventually retired for good. For now it still appears in many of our longstanding APIs.
The ultimate goal is to refactor each one of those APIs to use just regular java.util.Map or one of its implementations. However, the scope of that refactoring is very large. And there are some things that ParameterList does that java.util.Map doesn't that we would need to find/build replacements for. A notable one is the ability to store multiple entries with the same key.
When we eventually do the work to retire ParameterList it won't be without some kind of notice. I should also add that it will likely be easy to retain some kind of ParameterList class so we don't break the thousands of extensions currently in use.