Currently the range for the uplift slider is 0 - 100%, the one for the confidence level slider is 80% - 100%:

Technically not all of these values can be supported as some of them would require sample sizes to go towards infinity. Once we conclude ABTEST-450, we need to review the permissible ranges and adjust the sliders on the UI as necessary (e.g. uplift 2% - 100% and confidence level 80% - 96%).
Update: the permissible ranges are documented in the SampleSizeCalculator