As the parent says, the API works fine as is, today, on 32-bit devices - you're just not allowed to submit code to the App Store that takes advantage of this. There is no JIT or anything else inherently architecture specific in the WebKit content blocker implementation. Thus the restriction comes either from a semi-arbitrary determination of the speed of Apple's first 64-bit processor, and/or the amount of RAM that came with it, as the boundary below which content blockers might cause unacceptable performance regression (despite their /improving/ performance on many sites)... or from some odd marketing plan to sell newer devices. Hard to say which.
Personally, I doubt it's a marketing thing. Apple do sometimes restrict software features to specific devices (Siri when it launched originally), but this is rather oddly processor-dependant. So I assume it's a performance thing.
