Are there any best practices on affixing navigation for a deep hierarchical page? Specifically, is it ok to affix just the sub navigation while leaving the global navigation at the top of the page? The goal of this is to simplify things for the user so that they're forced to focus on the current page. This is an example of a website that does it.
Your example has one minor flaw. The responsive version of your example looses the subnavigation and its intended outcome.
If the global navigation is already conjested how do you find real estate for the subnavigation? Do we hide it on mobile devices?
Consider this. If the subnavigation does not migrate to the slide in menu. If the sub navigation becomes a list underneath the landing draw. How does the user get back to the sub navigation if they have been transported a distant fold of the page. Scroll back up? When you are making this decision think about the mobile user.
Here is my example:
I would say that it is a common practice for a page with deep hierarchy or complex IA to have a navigation menu that is independent of the rest of the navigation structure. It does depend on if it is a one-off page or something that happens for a lot of the pages (in which case you might need to think about restructuring content for easier consumption by the audience), but if you make it consistent then there's not going to be much confusion for the user either way.
This sub navigation can be like a table of contents for the page that is at the top of the page (e.g. Wikipedia) or fixed to the left or right hand side of the main content (e.g. GOV.UK Service Design Manual).