To me, the question is, does the current AM_WITH_DMALLOC actually work? Or not? If it does, I see no reason to have it issue a warning, and thus annoy anyone who has it in their configure.ac, where it is presumably residing harmlessly.
I agree it does not "fit with Automake's mission", and wouldn't want to add any similar macros, but to me that's not enough reason to "deprecate" something that's been part of Automake for so long. I feel strongly that the macro should never be deleted, useless though it may be(come). -k