So yeah, there is a literal void you can enter in the game (and fish in)! While it looks like a glitch/bug, it actually seems to be completely intentional, as a message will display when you enter the void, and when you leave it. So, how did I do this?

Step 1: Private Island Prop

Stand near the pictured wall in the main building. Looking towards the wall, place a private island prop.

Step 2: Enter Private Island, Then Leave

Yeah, do that. Travel to the island, then leave the island.

Step 3: Proto-Void

Where you will be at this point is the proto-void. No messages will display, and you will be able to see inside of the main building, but I believe fishing inside of it still acts like the void.

Step 4: Void

Walk/Sprint into the proto-void until you fall. At this point, you will respawn in the void, and see this message in the chat.

(I got rate limited , the message says: "you feel a light breeze")

Step 5: Back to the Light

By walking/sprinting into the real void, you will eventually fall and respawn back into the main world. The message shown will be displayed in the chat.

(Still rate limited, the message says: "you feel warm once again")

That's it! I've heard rumors from my siblings that there's a special fish that can only be found within the void. It requires higher-level bait (I think), so I will let you know if it actually exists. Or there's probably an article about it on one of those gaming news/guides websites.