How to create a mod in an old version?

no, sorry you can't mod for 1.7.10 anymore. it was in older MCreator versions, but is not supported anymore

1.7.10 is outdated and there is no software supporting it

I see that i'm 2 years too late, but, I actually found another page that asks a similar question or, well, provides an answer to the question of wanting to make mods for older Minecraft versions.

So instead of just saying "It's Out Of Date", and saying you can't support your wish to make older modded Minecraft versions, I understand the reasoning and value to why a person would want to go out of there way and would like to make a mod for older versions of the game.

You may want to try and make your mod for as many different versions of the game as possible for maximum compatibility across the board. So that people may enjoy the most of the experience that you and your mod provides. Or maybe there are some old mods that don't exist in newer versions of the game, and you wanna make your mod for the older version cause they have additional mods there you want to make some along the side with. Or for any other reason or purpose.

So, this is a link to a forum I found where someone actually discovered an old archive of all of the older Minecraft versions from Minecraft versions 1.4.5 all the way to 1.7.10, and that last one is the version you wanted to make mods for.

Here is the link to that page - https://mcreator.net/forum/55550/mcreator-older-versions-archive 

Also, here is that link that the page provides that leads you directly to the archive of older MCreator Versions - http://web.archive.org/web/20170916180734/https://mcreator.pylo.co/download/old