Is it true or false that each flow runtime (Classic, Lightning) is restricted to its respective desktop experience?

The statement is false. In Salesforce, flows can run across both Classic and Lightning experiences. While certain features or interface elements may differ between the two platforms, the capability to use flows is not strictly limited to either environment. This flexibility allows users to create and deploy flows that can function in both Classic and Lightning contexts, thereby enhancing the usability and versatility of process automation within the Salesforce platform.

Other options suggest limitations or one-way compatibility between Classic and Lightning, which does not accurately reflect the reality of Salesforce flow deployments. Flows can be designed in one experience and utilized in the other, promoting seamless integration between the different user interfaces within Salesforce.