Is it correct to write "25% off all subscriptions"?
Why are we omitting on here?
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You can use on, inasmuch as all subscriptions have an offer on them (There's 25% off on all subscriptions) but it's unnecessary and introduces an extra hop in processing the sentence.
Instead of Nigel J's fairly straightforward ellipsis of what off refers to ("There's 25% off [the price of] all subscriptions"), you are adding what on refers to as well: "There's [an offer of] 25% off [the standard price] on all subscriptions".
It doesn't mean it can't be done, and it's still right to do so, since it is merely another ellipsis; but the additional ellipsis makes the sentence more complicated.