Make field visible in project, but leave child projects mixed

[Matt Seitz]

How do I make a field visible in a project, but leave its visiblility mixed in child projects?

I thought previously the "Edit Field" screen had 3 options for child projects: yes, no, or mixed.  But now I only see 2 options, yes or no.

[Cali McClung]

Hi Matt,
Yes I recall the Mixed option too and it's still shown in the Help pages.  At least you can see the number of Visible and Hidden child projects below that. 
Now you can get more information using the new Custom fields usage report. Click on the Export custom field usage for workspace.  The export show all the projects where a mixed field is visible.