It was marked as "deprecated" in MW 1.21 as part of the CH cleanup in T145728, this seems that although it was right, it was also wrong
Just to summarise what was said in IRC, I think this function was deprecated in error. WikiRevision is supposed to be a thin wrapper for the XML dump <revision> tag, and the $text field stores the raw content of the <text> element. Nothing scary at all about it.
@TTO ah, I see. In the context of WikiRevision, it makes sense to allow access to the raw data. I suggest to rename the function to make clear that what you get may not be "text". It may in fact be binary garbage. So the function should be called getData() or getBlob() or getBytes() or something like that.
Direct access to the $text member should be avoided, I suggest to change all direct access to using the getter, and to make the member private.