GO Transit buses and ticket counters at GO Stations no longer take cash (don't know the date of the change). You can use cash at the fare vending machines (I assume all machines now take the newer $10 bills but if not, make sure you have other bills or coins on hand to ensure sufficient fare payment).
https://www.gotransit.com/en/trip-planning/fares
Since banks can charge businesses for deposits, the change likely makes banking/coin handling less necessary. Bus drivers must be thrilled not to have to dispense change.
Can use a Presto card if you can load it online or in advance (ie. Shoppers Drug Mart).