This is a dumb question, but what is the separator plate for? What does it separate? Why did Subaru use plastic ones?

The answer to why is always money. As to what for, I don't know, always guessed it covered a spot where it simplified casting or machining that side of the block? In that case, why not one on each side?

I saw someone say it is an access port for engine service or something.

 

I saw someone say it is an access port for engine service or something.

 

 

There's a baffle on the plate the would be difficult or impossible to confirm clean or inspect if it cast into the block.

It's basically there to cause leaks, though it's main purpose is to separate your money from your wallet.

Call it mechanical apartheid. A very dark and not very well known segment of Fuji Heavy Industries history. But even well hidden and shameful things have a tendency to leak out.