Changing the return type of getUsedHeapSize() etc. to long breaks binary compatibility of user written plug-ins, but will be a trivial/no source change and recompile to use the modified method in a 1.0 version of MAT. You could write code which would compile with either version, but it would not be binary compatible.
Adding a getShallowHeapSize() method and deprecating the old method keeps binary compatibility for existing user plug-ins, but to take advantage of the changes requires a recode to change uses of getUsedHeapSize() to getShallowHeapSize(), and then the new version won't work on old MAT 0.8 either. It also clutters the API.
Would writers of plug-ins for MAT like to comment?