In general, its behavior is dependent on the Unicode version (as new codepoints get added and case-mapping tables change).
Since the function was moved, you get a deprecation warning for now. In 1.0, it will just be an error. You will just have to do using Unicode.
I agree that there is a tension between the convenience of the “batteries included” approach where everything is in Base (but then can’t be updated separately from Base) and the “modular” approach where you need to import a bunch of modules to do anything nontrivial (but upgrading/compatibility/support is easier). As more things get moved into modules, I’m sure all of us will feel annoyance as something we personally use goes into a module. Hopefully a good balance can be struck.