Title::getUserCaseDBKey() is used to get the db key, with the initial letter casing as specified by the user. In core it's only used in FileRepo.php and MediaWikiTitleCodec.php, and has a minimal number of uses in extensions (CentralAuth, PushToWatch etc).
The main problem with this is that depending on how you construct the Title object, this function may or may not work. It's also one of the differences between Title and TitleParser, and I don't see any good use cases of keeping it around.
- Soft Deprecation
- Removal from production
- Hard Deprecation
- Removal of code from code base entirely