In the latest update of Azure DevOps, we’ve added user assignment-based billing. With this feature, the number of Basic or Basic + Test Plan licenses will change as you add or remove users. This means that you’ll only pay for the licenses you’re using. We’ve also added a new setting that lets you choose whether new users get full Basic access or limited/free Stakeholder access.
See the list of new features for more information.
Source: Azure Roadmap