is_expired only exists in the 4.x branch, specifically since 4.00_01. There is an _is_expired in the 3.9x branch, however, you're unlikely to ever have a case where you'd use it since 3.9x creates a new, empty session for you if an expired one was already found. You can check for this condition with is_new.

Note, if you want to get a session and guard against the recreation of the session, use load in the 4.x branch.