Actually, the apostrophe here is correct. It's possessive. The 's' is not plural.
'Your Classic's', is just a shorter way of saying 'Your classic flavour's original taste'. It's clunky but it's grammatically correct.
@red dragin While it might be technically correct, it's an awkward way of phrasing it because they've truncated the sentence to make it fit.
They've also used 'one of your' which makes the plurality of flavours both singular and possessive in a way that doesn't intuitively make sense. Multiple flavours would generally be plural, and 'your classics' would be the plural way of referring to the group of flavours.
The addition of 'one of' and 'your' confuses whether they are referring to one 'classic' flavour or 'one of the classics'. Since there are multiple flavours the sentence, as written, would read correctly as plural rather than possessive singular - without any additional words being truncated.
The truncation of those words makes the apostrophe confusing because they're referring to a singular, possessive noun, but your brain has to figure out what words are missing since the sentence would make perfect sense without any more words if it were plural.
TL;DR it's just bad copy writing and they should have found a better way to make their point that isn't grammatically awkward.
I hope this helps and doesn't just make you more confused.