When you're done with your experiment, you can now chose to ship it with an experiment-specific holdout. This is helpful when you're done with the test, are shipping a test group, but still want to measure impact on a small subset of the population to understand longer term effects.
Example use case : When ending a 50% Control vs 50% Test, you can ship Test with a 5% experiment specific holdout. Statsig will ship the Test experience to 95% of your users - and will continue to compute lift vs a the 5% Holdout. It compares this 5% holdout (who don't get the test experience) to a similar sized group who got the test experience when you made the ship decision. You can ship to the holdout when you conclude this. See docs
Statsig also natively supports Holdouts. These typically are used across features, and aren't experiment specific.