After hours of wondering why my CSS was still being purged even though it was inside /* purge start ignore */ and /* purgecss end ignore /*, I discovered a simple answer on Github that I wanted to share.
Add a ! so your ignore comments look like this
/*! purgecss start ignore */ /*! purgecss end ignore */
This removes any comments in and around rules, selectors & declarations. But comments marked with ! are kept by default.