Standalone components are not mandatory and will never be, there is no rule when to use them. However, Angular Architects recommend to always use Standalone components at least for new components you create. They are simply more treeshakeable and less boiler. You can mix standalone components and modules also.
For the mentioned recommendation of the Angular Architects, you can also watch the Webinar from last Monday: https://www.youtube.com/watch?v=9rj8kR0q0c8