I FOUND IT!!!
https://i.postimg.cc/BvBWRf9Z/tsb-golden-goomba.png
Here's how you find the easter egg:
Go to Route 1-1 (SMB1 level in the first world)
Go down on the second Warp Pipe to access the bonus room
Break the bottom brick blocks (there's a hammer inside an invisible block at the right)
Press down at the center of the floor (you have to be pretty precise)
You will warp down and discover the golden Goomba!
Like Thunder Dragon said, there's pretty much nothing to do at this point other than letting the golden Goomba kill you. It just walks back-and-forth (like an ordinary Goomba), and your attacks do nothing against it. It also sparkles! The golden Goomba doesn't instantly kill you like the TD statue does, but instead takes out one hit point at a time.
I found the location of this easter egg by extracting level maps of the Demo V3 version (https://web.archive.org/web/20051020142152/http://mfgg.net/files/games/tsb_demo_v3.zip) (from early 2004) via ClickExtract. Once I looked at the level map for this room and saw the golden Goomba down there, I thought "This must be it." I spent a few minutes in there trying to find a way down, then I looked at the map again and noticed that along the middle of the room are Toad markers that are used for when Toad enters and exits a Warp Pipe. Soon, that lonely Goomba finally had a buddy to play with after about two decades! (Their playtime didn't last for very long, sadly.)
Interestingly, you could kind of tell beforehand that there was something below in the bonus room. If you look down, you'll find empty space below the blocks. Though, I assume that if anyone did so, they believed that there's nothing down there. Then again, was there any other part in TSB where you can see empty space beyond a wall/floor/ceiling? I know there's the Donut Block plugging the shaft in the World 7 hub, but I believe TD said in the IPB forum thread that there's nothing down there, it's just reflecting the design of the final level from SMB3 (which is something I've never noticed until then).
It's really cool to find something that's been hidden for at least 20 years! Considering how simple it is to find, though, I feel a bit ashamed for using Click Extract. :whoops: But the discovery was exciting! Now I wonder if someone else is going to include an indestructible golden Goomba into their game, hidden in a way that is likewise ultra-obscure and nearly-impossible to find... or has it already been done?
