My input probably isn’t as valuable except as a data point, but the company I work for (around 100 employees) moved all employees over to a 32 hour/4 day work week without a change in salary. No negotiation involved technically, though there were a few employees who have been talking about 4-day work weeks for a long time.

They made this change with a 6-month trial period where we would determine if we would keep it at a later time based on some sort of productivity measurements. Everyone, including the CEO, seemed to agree that trying to measure productivity would yield few meaningful metrics. The consensus at the end was that productivity was either the same or a little less, but we ended up keeping it anyway.