GroupBy by nodeJS or ES6 of very nested JSON Objects with condition
A few thoughts;

Most would write const flatFeature = []; to initialize an array in javascript, though what you have is certainly correct.
You should strive to de-nest your code.  Having code unnecessarily placed in if or if-else blocks increases the number of paths through your code, making it harder to follow and unit test.  In this case, you could de-nest by changing your first if to something like if (!subscription.plan || !subscription.plan.feature) return; which clearly indicates that there is no work to do in this function unless those conditions are met.
You should strongly consider using an object to map your flatFeature names to the objects encountered with greatest expiration date.  That eliminates the looping find() operation to determine if an existing flatFeature with that name has already been encountered, making this a constant time lookup.  This may not be a big deal if there are 100 subscriptions being iterated, but what if there are 1 million?
Taking above approach of building a name-keyed object gives you flexibility to continue to work against those keyed feature names or you can simply use Object.values(obj) to get the array of feature objects.
Stylistically, your code looks somewhat &quot;dense&quot; and has long horizontal lines (especially your ternary) that make code harder to read and understand at a glance.  Using javascript's destructuring can oftentimes help in keeping your code more terse and thus more easily read.

Putting it together, I might use something like:
const features = allActiveSubscriptions.reduce(
  (obj, sub) =&gt; {
    const { plan: { featuresList }, expDate } = sub;

    if (!featuresList) return obj;

    featuresList.forEach( ({ name }) =&gt; {
      const existFeat = obj[name];

      if (!existFeat) { 
        obj[name] = { name, expDate };
        return;
      }
      
      if (expDate &gt; existFeat.expDate) {
        existFeat.expDate = expDate;
      }
    });

    return obj;
  },
  {} 
);

// if you want an array
const featArray = Object.values(features);

In terms of worst-case operational efficiency, this makes your function perform in O(m x n) as opposed to O(m x n x p) as it is currently (where m is number of subscriptions, n is maximum feature list size, and p number of unique feature names).