Part and function?
 

What is the 90 degree angled hose that goes from a nipple on the back of the block to a nipple on the throttle chamber? (It has coolant in it.)

Is it important? What is it's function? Would I be able to cap it off??

Cannot find pictures. I've looked through the service manual and cannot find it either...

i think it might be connected to the heater core don't quote me on this because I am not looking at the engine bay. When I took off the two hoses that were connected to my block and my firewall they were connected to my heater core and had coolant dripping out of them.

look up thermowax.
I am thinking there is a line that goes to the TB that is supposed to Eventually Open the Thermowax portion of the throttle body and it makes the car Come down in Revs as it warms up.
Wax heats up,car warms up,and idle decreases to regular Status(about 750 rpm).

Hmm.. So with a somewhat highly modified car, would I be able to cap it and forget it?

Yes. It's the crossover from the back of the water pump to the rear plate, you don't need it if you don't have a thermowax. You can still make the car idle cold with a working BAC.

Surprisingly, I have found more posts about that hose... I am SO confused on whether to keep it or cap it off. Many say to keep it, but some say to cap it off. I figured I would need to keep it though. Especially with the cold temps now...?

In Illinois? Yeah, I'd keep the thermowax and associated hoses. I have mine still in place and it helps even with "cold" starts here in California.

i like the fast idle in the morning vs stalling. i like to keep the thermowax. if you buy the 89+ thermowax, you can use the s5 hose which is 100000x easier to get on and off