What technique do API developers most commonly use to limit access to an API to authorised individuals and applications?

API keys

Explanation:

API (Application Programming Interface) is a computing interface that defines the interaction between multiple software intermediaries.

API key is a specific identifier use authenticate the developer user and call programs for the API.

The API keys often serve as a specific identifier and a secret token for authentication, and usually have a set of access rights to the API associated with it.

These are commonly used to authenticate a project with an API instead of a human user. Different platforms can execute and use API keys in different ways.