Ulrike and Hans wrote
It is not only that font. Actually the libertine package broke, fontawesome broke, and Coelacanth was only used by Thérèse in the example as it is free, her real problem was with using Goudy fleurons.
in context i strongly advice against using numbers instead of names
It's not just explicit numbers via \char, it's character data in documents using specific fonts with documented PUA characters Many fonts use this for assorted reasons https://en.wikipedia.org/wiki/Private_Use_Areas#Vendor_use notably SIL fonts using it for minority languages not in Unicode and Microsoft for all kinds of CJK stuff. Given how many reports are appearing in the few days that this has been exposed to a larger number of uses via LaTeX, use of PUA characters really isn't a rare occurrence at all. If you need to allocate a block for internal use wouldn't it be possible to use one of the high areas Supplementary Private Use Area-A or B (U+F0000 - U+FFFFF) (U+100000 - U+10FFFF) ? The BMP PUA block (U+E000 U+F8FF) just has so many documented uses in existing fonts. David Disclaimer The Numerical Algorithms Group Ltd is a company registered in England and Wales with company number 1249803. The registered office is: Wilkinson House, Jordan Hill Road, Oxford OX2 8DR, United Kingdom. This e-mail has been scanned for all viruses and malware, and may have been automatically archived by Mimecast Ltd, an innovator in Software as a Service (SaaS) for business.