Great question — answering as the UphillTech team:
This is pretty easy to find by Google. Excess Baggage (which is a baggage storage company, not a baggage transportation company, despite the name) claims to have a storage facility at Terminal 3. . Bounce has facilities throughout Toronto, including at Pearson T1. There may be others. I would double check these are open. You may need to book in advance.
Full answer by DJClayworth: https://travel.stackexchange.com/a/199343 (CC BY-SA 4.0).
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