The idle speed dropped from 1100 to 5 or 600 rpm, barely holding it. Not progressively, no; suddenly. From one traffic light to the next. I checked all connections between spark plugs and distributor. Everything looks fine.

In order to drive the car, I re-adjusted the idle to 1100 rpm with the idle screw at the carb but I know I am just compensating for / hiding a problem.

I checked the vacuum line between the carb and the distributor. When disconnecting it (and plugging it with my thumb), the idle speed drops a little, which tells me the canister works.

At this point, I am puzzled. Any suggestion?

5 might be a tad low. 7 would be nice. 11?? Quite high. Why? 

Spark plug wire, different vacuum hose, something in the distributor/condenser.

1100 rpm is the manufacturer specification for this engine. I know it sounds high, but this is what it is. And 700 does not sound right, believe me.

Check ALL of the vacuum lines coming of the engine first...not just the one to the dist...which wouldn't have that much effect on the idle speed anyway. Holley or Quadrajet carb? Possible the throttle blade shaft(s) wore, and the thing just closed more than normal. Happens more than you think, even on fuel injected engines. 

Did anything happen right before this started? Like a backfire?

If all checks out, check timing. Check balancer condition.