I am 👎 on this change. Maintaining BC for protected properties is a nightmare. And you can already achieve the same by overriding thegetProjectDir() method in your custom kernel class.

@leofeyer allowing to override these variables is very important ... but we don't allow to do that changing the value of the properties directly, but by overriding thegetProjectDir() method of the Kernel class, as@xabbuh explained.

So I'm going to close this issue as "won't fix" because this can already be solved in other way. Thanks!

Note that writing directly torootDir is not really something we want to support either (you should also overwrite the method). But we cannot switch it to private outside major versions

thus, you still have not provided a use case requiring the visibility change (i.e. something whichcannot be achieved by overwriting the method)

Ok, thanks for the clarification. We should merge#22728 then.