Are bear canisters required for backpacking?

It depends on the area. Many national parks and wilderness zones (notably Yosemite, parts of the Sierra, the Adirondack High Peaks, and much of grizzly country) require an IGBC-approved hard-sided canister. Specialist communities consistently note that canisters are the only food-storage method allowed everywhere, while bear bags and Ursacks are restricted in some jurisdictions.