I've just added a new status which can be applied to phabricator tasks.
It is now possible to explicitly mark a task as "In progress" to indicate that you are working on it. This is in response to some Wikimedia teams using a workflow that involves assigning tasks before work begins. Once work begins in earnest, you can now make that status change explicit by marking the task with the "In progress" status. Functionally it's identical to "open," however, this will allow for some improvements to progress tracking and reporting.
Feedback about this change can be submitted to Phabricator on the task[1]