As per my understanding eVars are persisted for the duration of a visit so that it can be later associated with any events. Where as lifetime of props is just that image request.
Just wanted to understand what is the scope of s.channel variable? Is that value persisted across multiple requests?
You're correct that props don't persist, and eVars can persist depend on their settings in the Adobe Analytics Console (default is visit, but they can also be set to expire on hit, or after a day/week/month, or never).
The s.campaign variable is treated identically to an eVar. You can configure how long s.campaign persists. Other variables behave just like props, like s.pageName, s.channel, and s.server. So to answer your question, s.channel behaves like a prop and expires on hit.