Why it is impossible to install old mods on new versions of minecraft?

[BOLNICHKA39]

Sorry for bad english). I really love minerakraft, and I like that the developers add new features to the new versions that I want to have, but there are a lot of mods that will never be released on new versions of minecraft, for example golem mod for 1.5.2, I really love these dudes, but they are not on minerakrat 1.6.0 and above. Is it really not possible to install an old mod on a new minecraft?
 

[MattieBit]

It isnt possible unless someone was to make an updated copy because the new mine craft versions have differences each time a new one comes out this means that the old code might be able to run but it could cause serious issues which is why forge does not let you so it can not cause any issues at all.

[larsgerrits]

Mojang doesn't write Minecraft with mod support in mind. That means that the code can change (a lot) between versions.

 

Mods use that code to add new features. If the methods they use get removed between versions, that mod doesn't work on a newer version of Minecraft because it can't find that code anymore.

[PolygonFruits]

This is actually what is happening currently with 1.12 >>> 1.13, and the same happened from 1.7 >>> 1.8. Mojang rewrites parts of the code that mods rely on, therefore needing an update. The thing special with these two instances is that Mojang substantially changed code(The Flattening in 1.13, I don't remember what happened in 1.8).

[MoeBoy76]

1.8 was the removal of numerical IDs in favour of names

[Cadiboo]

Are we forgetting 1.7.10 -> 1.8? The entire rendering system changed completely.

2 hours ago, PolygonFruits said:

The Flattening in 1.13

The flattening itself isn't a problem, its a code choice made by Mojang now that they've (finally) expanded the block ID limit and abstracted away metadata. The real change with 1.13 is Terrain Gen.

 

Also in 1.13 Forge has rewritten its entire mod loading system and should now support Java 9+

[V0idWa1k3r]

There was also the 1.2.x -> 1.3 transition when Mojang "merged" the client and the server code. There was no "common" code in 1.2.x, period. If you wanted your mod to work for multiplayer you had to write a separate version. I think there even was a separate  API for multiplayer mods - Risugami's ModLoaderMP(although it might have became obsolete earlier)

[Anon1234]

I completely understand the code rewriting on making previous mod versions obsolete, but if you know what code is changed you can just code Forge into converting the code from, say a 1.7.10 mod using the 1.7.10 Minecraft (MC) and Forge code, into the code used by the newer MC and Forge version. It's not impossible, and yes I know that would take a massive amount of time, effort, and accuracy before anyone gets triggered due to this probably being asked many times, but the team pretty much goes through that with rewriting the code every MC rewrite and mods use the combined MC and Forge code. All that's really necessary is to compile (non-coding definition) a reference library or something along the line listing the conversions from version X MC and it's corresponding Forge version, to version Y MC and it's corresponding Forge version so mod creators would be able to more easily port their mods from one version to another.

[DaemonUmbra]

This is not going to happen, locking