Register Icons BEFORE 11
nice example in ext_localconf.php: https://github.com/benjaminkott/bootstrap_package/blob/master/ext_localconf.php
Icon API changes in TYPO3 11 using Configuration/Icons.php
https://usetypo3.com/icon-api.html
TYPO3 Icons
https://typo3.github.io/TYPO3.Icons/index.html
Plugin Icon
https://docs.typo3.org/m/typo3/book-extbasefluid/main/en-us/4-FirstExtension/7-configuring-the-plugin.html?highlight=icon#registering-the-plugin
Icon API
https://docs.typo3.org/m/typo3/reference-coreapi/main/en-us/ApiOverview/Icon/Index.html