Supposedly McDonald’s is the only business that gets the syrup distributed in metal kegs. Everyone else gets the plastic bags, which could lead to some taste difference.
I am pretty sure I could blind taste test canned vs bottled soda.
AFAIK they are completely phased out in the soda industry (including McDs). They are very popular in the home beer brewing community because they are easy to clean and the right size for your typical homebrew batch (20 litres). They've become difficult to obtain in recent years because they are not being used in industry any more so there's no secondary market.
Some players are making them new, but they are quite expensive. I used to have a couple of regular beer kegs on "long term loan" from Heineken because I was not able to source cornys for cheap.
I am pretty sure I could blind taste test canned vs bottled soda.