At the moment we use Typo3 version 4.7.19 on core, mostly for charset and encoding stuff, see lib/classes/text.php.
However that version is out of support and would be expensive move our code to use the latest version. The Typo jumped from 4.7.20 version to 6.X version and there were lots of structural changes and lots of legacy code of the 4.X version that got refactored.
The goal of this issue is to verify the possibility of replace our Typo3 library by PHP iconv functions.
A bit of history about this Typo 4.7 to 6.0 version jump: https://typo3.org/news/article/from-47-to-60-a-brief-history-of-typo3-versioning/