As of today, API v1 is deprecated. In order to provide our customers the best possible features and a single, consistent REST API we will turn off API v1 on January 1, 2022. API v1 has been serving for nearly 8 years now and it is time to move forward.
API v2 was released for production use earlier this year. With the recent addition of the Get Metadata API, all features of API v1 are now available in API v2. API v2 uses a more robust design and better extensibility for new features. You can read more about the fundamentals of API v2 here.
First of all, you want to build all new apps with API v2.
Second, you need to update your existing integrations to API v2. API v2 is using a completely different design, so please check out the API docs to get familiar with it. You have more than one year time to transition your app to API v2, which should be suifficient.