70's does appear to be contraction, and therefore missing elements ought to be represented by an apostrophe.

Even if this is not absolutely correct, it is usually obvious in everyday usage that we are referring to the decade between 1969 and 1980. It is tedious both to write and read "nineteen-seventies" everywhere. Even in more scholarly works, I believe only dedicated pedants would cavil at the usage e.g. "1670's".

I agree that 70s doesn't look quite right. Seventy shillings ? A gigantic pair of trousers with short inside leg ?

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I take the view that, if someone thinks and communicates logically, minor slips of punctuation (or typographical errors) will not change or obscure the meaning of what is written. The sentence, "Eats, shoots and leaves" might be incorrect and even raise a titter; but where in the real world do pandas carry firearms ?