Series voltage reference output voltage
If you can't find the answer in the data sheet then you can't make any assumptions about what will happen. You might get zero volts as soon as the input voltage falls below the minimum specified value, or the regulator might give some output voltage that is close to the desired value. The behavior may be different for different manufacturers of the same basic part number, and it is likely to be different for different kinds of regulators.

Some readers here will tell you about their experience but you can't count on seeing the same thing in your application with your parts. The datasheet is the only authoritaritive source.