@julian @psychobunny I really feel this plugin is important. It makes total sense for a community admin to support their users in a private place which can be tracked in a scalable way.
It has numerous applications from IT support portal, learning communities , healthcare communities, ombudsman group in enterprises. This can bring you lot of customers who looks for 1 stop shop for all community managment. You can highlight this feature in your site as well.
I follow Discourse and have seen them implementing it very recently among lot of requests (In fact they have done it in a complex way within groups functionality). Nodebb plugin approach of using categories IMHO is the right way.
I understand that you would love to do it as a custom service. I feel you would get more visibility if you do it as a community plugin. Since you have already have it for older versions, I feel it is wise to do it for the new versions as well. I would urge you to reconsider the upgrade path for the support plugin.