How to use notifications for a custom model?

I have a custom server model in a plugin. When a user interacts with that model to create or upgrade a server I want to send a notification when it’s done. The notification model has:

  belongs_to :user
  belongs_to :topic

I’ve tried creating a notification with some data, but even following examples in existing notifications can’t make sense of what can be in the data field (not to mention anything useful).

Can someone point me to a plugin with an example custom notification?

Also, there are comments suggesting that the custom Notification identifier is troublesome and that those who want custom notifications should submit a PR asking for one, is that still true?