I would like to submit a request for the revaluation of removing the SSRS Broker from “legacy” status back to active support/development. There are still many on prem customers that use SSRS including my current customer.

This could be accomplished in a few ways that would be of benefit.

Option 1: Update the current broker project. This is the quickest and low hanging fruits. The necessary web services that the current broker reply upon exist in SSRS 2016. This has been validated in a fresh build of a test environment.
Change the reference from depending on the ReportViewer 2005 to Report Viewer 2016.
Update the project .NET Framework Version to one that is more current.
Recompile and remove the legacy label.
Option 2: Removed the report viewer dependencies and update to use the new Report Viewer Controls Nuget package.
Package manager can be used to add new DLLs to the project for the most recent version of SSRS / Power BI which is still backward compatible back down to SSRS 2005.
Update name spaces to reference the same methods, properties and types used today in the SSRS broker to leverage the new viewer control dependencies.
Option 3: Consider releasing the broker code as an open source project so that the community could enhance the broker beyond feature parity that the first 2 options offered aim to accomplish if Nintex doesn’t have an interest in adding net new functionality.