A developer is creating a class \Vendor\Module\Model\MyModeL How should that class be defined as transient in di.xml?

To define a class as transient in Magento's di.xml, the correct approach is to set the shared attribute to "false" for that class. This tells Magento's object manager not to use the singleton pattern for this class, meaning a new instance will be created each time the class is requested. This is particularly useful for classes that should not maintain state across different areas of the application or during a single request lifecycle​​.