A seam that made a deletion possible

An interface introduced in 2022 for testing, which turned out to be where a module could be cut out.

interface SubscriptionEngine
{
    public function entitlementsFor(Customer $c): Entitlements;
}

// one implementation for four years, and when the
// product line was discontinued the whole module came
// out behind this one method — replaced by:

final class EverythingEnabled implements SubscriptionEngine
{
    public function entitlementsFor(Customer $c): Entitlements
    {
        return Entitlements::all();
    }
}

A seam created for a reason that turned out to be secondary is the usual story with the interfaces that matter — the tests were fine either way and the deletion would have been a fortnight without it. It is also unfalsifiable as a design principle, because the four other speculative interfaces from 2022 never acquired a second use.