Early on in the history of photography, acidic developers (based on iron, copper or other salts) or ammonia developers which used ammonia as alkali and solvent both, were the favorites of many photographers. Even into modern times, workers researched acid developers and I remember reading many patents on exotic or not so exotic developers that used odd organics or metals or metal complexes to develop photographic materials.

AFAIK, they all fell out of favor for one reason or another, or simply vanished from the landscape except for the acidic amidol developer. It too has a major drawback in the sense that it is very unstable once mixed, but then most all amidol developers are not all that stable.

PE