If implemented like HN x-id's you could just refresh or reload the page in order to get a new session. And like HN one could just expire a stored session after x minutes without being used.
Note though: Originally when I posted the session-id idea I was thinking tryarc was only one session with many threads, where each session-id would really just point to its own namespace. Had this been the case it would just be the functions and variables being stored in memory. And then threads/session-ids/namespaces (whatever) could be spun up or down and managed in a myriad of ways.
You: "If implemented like HN x-id's you could just refresh or reload the page in order to get a new session."
That's the behavior I expect, but then I also expect to get a new session if I navigate somewhere and click the back button. If the back button case does not give me a new REPL, I don't see why refreshing the page would.