Reverse-engineering a vintage comparator chip

I don't think the word "unused" is appropriate.

1) Probably they wanted to physically separate the comparators, for better noise isolation between them.

2) the large metal area is connected to V+. By using such a large amount of metal they get slightly lower voltage drop in their power distribution.

3) But here's what I think is the main reason. The metal is being used as a power decoupling capacitor. I can't be sure of what's underneath, but most likely the area is connected to V-. Voila! You couldn't possibly get a decoupling cap any closer to where it's needed than that.