Some people expecting simply too much of typo3-rector. To avoid these high expectations this section in the documentation exists.
At the moment typo3-rector is not able to refactor the following things:
- SignalSlots to PSR-14 Events
- eID to PSR-15 Middleware
- ObjectManager to PSR-11 Dependency Injection
- $GLOBALS['TYPO3_DB'] to Doctrine DBAL (only a few simple cases)
This list does not claim to be exhaustive. There are certainly many other things that typo3-rector cannot yet take on.
Have a look at all the currently available rules