What problem does this feature solve?

If I have a computed property with only getter, setter is set to a noop. If I then try to assign to such a computed property, nothing happens. This can hide programming errors.

What does the proposed API look like?

I would suggest that when not in production, instead of a noop a warning function would be assigned to a setter and a warning message would be printed if one tries to assign a value to such computer property.

Alternatively, output of computed property without setter could be configured without a setter at all, as a read-only property.