@rickkas7 gives a minimum version at Problems turning on -Werror. Thanks!
I would like to know what sort of soft-guarantees could be made about future breakage. For instance, if I introduce variables into that Makefile, is there a prefix we can use which particle promises to reserve for forever? I know I can do something complex like KENZ_KRAZY_KEYWORD_PWD ?= /home/kenz, but if Particle promises to reserve something specific (USR_?) then it can unit test against any accidental use of that.