TYPO3 Core

A very simple solution for this issue is to use

\Normalizer::normalize();

in/before

\TYPO3\CMS\Core\Charset\CharsetConverter::specCharsToASCII()

Will fail if "intl" is not enabled, but that can be checked. Better use it when it's available then not use it at all.

Ah, there is a polyfill:
https://github.com/symfony/polyfill-intl-normalizer

A brute force removal of ALL nonspacing marks:

Transliterator::createFromRules('any-NFD; [\p{Mn}] Remove; any-NFC')->transliterate($subject);

https://www.compart.com/en/unicode/category/Mn

The character should probably changed to list only Latin combining marks.

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/69144

Patch set 3 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/69144

Patch set 4 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/69144

Patch set 2 for branch 10.4 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/70255