Full time and part time

I’m talking about in-house, working for a company - design firm, manufacturer, etc. Not freelancing. Has anyone tried to work at the professional level, for pro-rated salary/benefits as a part timer? e.g. doing the same job you currently do, but with 3/5 the project load. In other words still doing the same job to the same standard, but working on fewer projects and for proportionally lower pay/benefits. It seems like a great idea and there would be no downside for the employer or the employee. Is this really a foreign concept?