The proposal had been championed by Google's Dominic Denicola but had remained in an early stage of development, according to Mozilla. Denicola declined to comment on the proposal's withdrawal except to say it was a "difficult subject." In recent comments in GitHub repo, he said the proposal had met "significant opposition from within Google."
While Mozilla declined to speculate on why the proposal was retracted, the company hopes another effort is made in this vein. "What has been retracted is a particular proposed solution," Mozilla said. "We believe the idea of making promises cancelable, in particular making HTML fetch cancelable, is still very much desired. We anticipate there will be other proposals."