When an API gains a large user base, all observable behaviors—even those not explicitly promised—become part of its de facto contract, limiting future changes.
This means that as more users interact with a system, they start to rely on every aspect of its behavior, not just what’s documented. This reliance forms an “implicit interface” that restricts how you can change the system later on, even for internal implementations. Essentially, the implementation itself becomes the interface, making any significant changes difficult without breaking something for someone.