> Be a drop-in replacement for vanilla or other servers
It seems to me that unless it's a drop-in replacement its not a Minecraft server? Akin to how say an Uno deck isn't a drop-in replacement for a Hearts deck but still both card games but not both Uno decks.
Or is it just meaning that Pumpkin (besides the network) do things differently than vanilla and so you might not be able to open a vanilla created world using Pumpkin?
The only "complete" reimplementation of Java Minecraft that I'm aware of is Bedrock.
For example there there are tricks that allow you to delete bedrock blocks. Which then lets you either get onto the roof of the nether, or drop below the bottom of the world. Not all of these tricks will then work depending upon the specific minecraft server.
Another example is that in vanilla you can "bomb" people with experience orbs, the sheer number of orbs on the screen will grind their game to a halt since there are too many objects to track and render. Some minecraft servers work around this by grouping up experience orbs into a single bigger orb. That way you have fewer orbs on screen at once.
[1] https://www.youtube.com/watch?v=FLynwXDnETI [2] https://news.ycombinator.com/item?id=41798369