The new "trigger a build by project" endpoint in practice triggers entire workflows and not singular jobs. In other words, it does not support specifying a job to trigger via build_parameters the way the other API job trigger does.
It would be greatly appreciated if the new endpoint could be made to support build_parameters so as to target particular jobs and not entire workflows. Our primary use case for wanting to trigger jobs outside of a workflow context via the API is ad-hoc mobile app submissions via our Hubot app in Slack. I imagine there would be other uses - there are plenty of opportunities for CircleCI-powered automation outside of a workflow context.
This would be especially helpful given that the other API job triggering endpoint is not supported when using the 2.1 configuration.