The spring still has to compress for the shackle to move back.
The shackle angle does affect spring rate. Having the shackle above the spring and pointing back (on the rears) gives a somewhat softer ride due to the leverage effect you're thinking of.
If they were angled forward it would be stiffer since the weight wants to move the shackle forward against the spring which wants to get longer and move the shackle back.
This assumes the shackle is above the spring, not the stock configuration which is what I think you're asking about.
If the shackle is below the spring the effect is opposite. If it's angled forward at rest compressing the spring moves the shackle back in the direction wheel movement wants it to go. If it were angled back the weight of the vehicle would move the shackle forward while the lengthening spring would want to move it back.
Sic friatur crustulum
'84 GW with Nissan SD33T, early Chev NV4500, 300, narrowed Ford reverse 44, narrowed Ford 60, SOA/reversed shackle in fornt, lowered mount/flipped shackle in rear.