The Private Topics Plugin is brilliant for this, but has one important weakness: it can only allow a single user per Topic. This means that you can’t add other users to the Topic without revealing all the topics in that category to them.
The workaround is to convert that Topic to a PM, or if appropriate move it to a more open location. But it would be lovely to be able to specify other users (and/or groups) to be able to access that specific topic. Would require a pretty complex solution though, I think.
Using the PM system for this use case is quite powerful and flexible (and allows pulling in other users); however, it also has an important weakness: it is silo’d from the Forum proper. This is much more of a problem on smaller sites without dedicated people to handle the support workflow, as it tends to either over-notify or get buried.