I think the reason it's not there is it's not needed any more. When you view and assign workbook permissions in v9 you see the permissions everyone has from Project level. In v8 you can't see this.
Oh please, <insert your favorite deity here>, let the "Inherited" permission be stricken from thy mighty Tableau Server permissions settings!
It still exists, but the language has changed to make it clearer what is really going on. Inherit was always a bit of a misnomer.
Previously, Inherit was essentially a 'null' permission or 'not set here, look elsewhere'.
In 9.0 this is represented by not setting a value in the permissions rule (leave it grey instead of green or red). On hover the tooltip will read 'unspecified'