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.

Enjoy purging!