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

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