Et futurum is a mod that adds 1.8 features to 1.7.10
But there is a section of the EULA that says that you can't use Mojang's code in mods
"If you've bought the Game, you may play around with it and modify it by adding modifications, tools, or plugins, which we will refer to collectively as "Mods." By "Mods," we mean something original that you or someone else created that doesn't contain a substantial part of our copyrightable code or content"
The Meaning of Life, the Universe, and Everything.
I suppose it's possible, but in that case Forge is HIGHLY illegal.
Though I suppose I should say that it used to be. Now, using byte-code changes I guess they don't have to use any Mojang code at all. Although to do anything at all, they first have to reverse engineer the obfuscation, which I think is technically illegal as well.
The EULA needs to be read a bit more carefully - they prohibit you from including a substantial amount of their code in a mod, and distributing modded versions - meaning a Minecraft jar that has been modded, not just the modded class files. The only real question is how they define "substantial".
Even the simplest mods that I've seen contain parts of Mojang's code copied and slightly modified; for example, the method "func_151538_a" in this mod is exactly the same as the one in MCP except they replaced a couple hard-coded values with variables (note all the variable names - obviously not original code since I would use meaningful names, and in my cave mods I have changed them; my mods also make far more changes, leaving only the basic cave generation algorithm recognizable and I doubt you can copyright some for loops and random calls). You could also say that simply recoloring a texture, as many mods that add ores, tools, or other items do (including my own), is also against the EULA, yet I see it being done all the time.
Of course, I have a mod myself which not only edits base classes (widely said to be illegal by Forge modders despite not being much different than copying part of Mojang's code and altering it) but adds features from later versions to 1.6.4, including, yes, the 1.8 stone types, which even use the exact same block ID and metadata (though I do not plan to ever update it to 1.8, using the same ID as stone is very convenient since I do not need to check for multiple IDs when generating ores and other stone-replacing blocks). Although I did not just copy the BlockStone class from 1.8+; likewise, when I added bone blocks I copied the BlockLog class since I noticed they had the same placement methodology; my fossils and igloos are generated by using code to place each individual block, not NBT structure files.
Then again, there are the 1.7+ trees that I added by copying code I found on GitHub (a clear violation of the EULA as they are distributing the source), though I did have to make some changes to it to work with block IDs and data values, and added a few tweaks of my own (e.g. dark oaks use spruce wood and oak leaves, with the biome determining what oak saplings grow into, all-bark textured logs for branches, and do not destroy bedrock, which was a bug that Mojang did not fix until later on; my mods contain quite a few bug fixes which I took from code posted to the bug tracker and elsewhere, or in this case, added an explicit check for air or leaves when placing wood blocks. I even added a feature before Mojang did, caves surfacing in all biomes, but I simply allow them to remove all blocks, not just a select few from a list, and ravines are included).
I have at least tried to keep down the number of modified classes that are not my own (and even my "own" classes contain code based on Mojang's) by using some hacky code, such as making mob spawners drop "empty spawners" by checking for them in the Block class (already modified to add new blocks) instead of properly modifying BlockMobSpawner, same for getting Fortune to only work on crops if it is on a hoe.
Also, Mojang appears to acknowledge the different types of mods and does not discriminate between them, "pre-run", which I take to mean a precompiled modified class (like my own mods), and "in-memory", mods that are injected into the game at runtime:
Any Mods you create for the Game from scratch belong to you (including pre-run Mods and in-memory Mods) and you can do whatever you want with them, as long as you don't sell them for money / try to make money from them and so long as you don’t distribute Modded Versions of the Game.
The main thing is that you can not claim a copyright on your mods if they contain any original Minecraft code in them, which is what got Bukkit into legal trouble.
For another example of how the EULA needs to be interpreted carefully, it gives you the impression that it is illegal to make any money off of the game whatsoever - yet servers are allowed to make money by donations, paid access, or cosmetic perks, as Mojang has said elsewhere.
Ironically, they even say that you can charge for access to mods on servers, despite the EULA forbidding making any money off of mods:
So can I charge for my minigames or mods? Yes, so long as all players on your server have access to the features.
The Meaning of Life, the Universe, and Everything.
Et Futurum is legal and so is Forge
Minecraft's EULA says the general one rule is "you must not distribute anything we've made unless we specifically agree to it"
they define "distribute anything we've made" as
give copies of our Game to anyone else;
make commercial use of anything we've made;
try to make money from anything we've made; or
let other people get access to anything we've made in a way that is unfair or unreasonable
So the way they define it is not as you would define it in English. Generally all EULA or contracts define terms in a legal way to be used in the court. That means do not distribute anything we made doesn't mean same thing as in English. It means what the EULA defines as do not make money or abuse everything we made.
Furthermore they go even into more detail as what you're allowed to do
"Below we also give you limited rights to do other things but we have to draw a line somewhere or else people will go too far. If you wish to make something pertaining to anything we've made we're humbled, but please make sure that it can't be interpreted as being official and that it complies with this EULA and the brand and asset usage guidelines and above all do not make commercial use of anything we've made."
They added that clause for mods. They encourage you to expand on their game but not to use their work to make money or make other games or use Minecraft brand or pretend you work for Mojang/Minecraft/Microsoft.
"The license and permission we give you to use and play our Game can be revoked if you break any of the terms of this EULA."
"If you've bought the Game, you may play around with it and modify it by adding modifications, tools, or plugins, which we will refer to collectively as "Mods." By "Mods," we mean something original that you or someone else created that doesn't contain a substantial part of our copyrightable code or content. When you combine your Mod with the Minecraft software, we will call that combination a "Modded Version" of the Game. We have the final say on what constitutes a Mod and what doesn't. You may not distribute any Modded Versions of our Game or software, and we’d appreciate it if you didn’t use Mods for griefing. Basically, Mods are okay to distribute; hacked versions or Modded Versions of the Game client or server software are not okay to distribute."
Note that you can include SOME code as long as you don't copy paste a lot. So adding a function here and there is okay. Finally they reserve the right to define something as a mod or not. So even if you do crazy stuff that might seem illegal they can jump in and declare your work is a mod.
long story short YES Et Futurm is completely legal and so is Forge
Bukkit was a different story they were making money of Minecraft brand thus breaking the non commercial use of mods clause I mentioned above.