Its not the material of the canister that changed the flavor, its the type of sweetner. The gubmint in all its infinite wisdom ruled quite some time ago that no longer would cane sugar be used for soft drinks. Instead corn syrup would be used. This was due in large part to the corn lobby paying off politicians to get a guarranteed never ending and ridiculously large income stream. So now when you sip a coke or pepsi or pepper or whatever suits you, you can take solace knowing that the unrefreshing syrupy film and aftertaste left in your mouth is because of a bunch of scum politicians. You probably relate the difference being glass bottle verses plastic because the glass bottled sodas were old enough that they were only made with sugarcane, giving you fond memories of the flavor from the bottled sodas.
On a positive note, if you crave the good old days of that sweet refreshing crisp taste of a soft drink sweetened by sugar cane, Mexican bottled cola's are your answer. You can get pepsi, coke and variety of other soft drinks made with sugarcane and bottled in the old glass bottles if you get them from a manufacturer and bottler outside the U.S. This is also much easier than you would think. You can go to tijuana and stock up, or you can just swing by your local dollar general. Dollar General quite frequently has mexican made and bottled sodas on their shelf for cheap and they are just like the old days, crisp, refreshing, made with sugarcane and have no syrupy after taste!