Yes, this is a feature that would be very helpful. It could be a toggled overlay where you can customize what dates, like holidays or other team related down town are tracked.
Just looked through the application again. You can still filter by any user within "Timeline", "Lists", etc. Unfortunately "Workload" only let you filter by full users, not collaborators. This is a...
It looks like it is now under a "Workload" top nav. - unfortunately it looks like they also removed the ability to filter by anyone who is not a paid user. Removing functionality is terrible.