I would like the ability to be able to track if a task/time entry is billable or not for any given project that task/time entry is related to.
If the project is billable per hour, the default would be on, and the user would be able to turn it off if they didn't want to bill the client for the time.
This would help keep all work aligned with the single project instead of having to create "a separate project for non-billable hours under a client so they can keep track of time without actually invoicing for the hours."
I've attached an example (source Clockify)