Data science Product Question Help

I was recently asked this in a ds interview pertaining to product thinking -

If a product feature is being rolled out, but an a/b test is unable to be performed due to whatever reason, how can we measure the efficacy of this feature?

My response was along the lines of an exploratory standpoint in terms of comparing data pre and post rollout, but was curious as if there were better methods for this, thanks and thanks so much for even looking at my question!