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A general description is here.
The character-classification functions used (by default) to replace the system ‘iswxxxxx’ functions on Windows, macOS and AIX have been updated to Unicode 13.0.0 - in particular, many more UTF-8 characters (such as emojis) are regarded as printable.
The character-width tables have been updated to include new assignments in Unicode 13.0.0.
The code for evaluating default (extended) regular expressions now uses the same character-classification functions as the rest of R (previously they differed on Windows, macOS and AIX.)