We've just started rolling out the ability to apply targeting on Holdouts. Holdouts work by "holding-back" one set of users from testing and comparing their metrics with normal users. Statsig now lets you apply a Feature Gate to your Holdout. e.g. if you wanted an iOS User Holdout, you could apply a Feature Gate that passes only iOS users.
Holdouts are the gold standard for measuring the cumulative impact of experiments you ship. (Learn more)