MCNostalgia Tool is a tool that takes binary difference patches and applies them to the latest version of Minecraft, essentially reverting it to an older version, without ever distributing any .jar files illegally. Binary difference patches are made by taking the differences between two files, one is the source/old file (in this case the latest version of Minecraft), the other is the output/new file (in this case an older version of Minecraft). To patch the files, MCNostalgia requires Minecraft to be unmodified, that means no imported textures even.
MCNostalgia Tool Installation:
Changing Verbosity Settings
Currently looking for somebody to do a video tutorial of using it on linux, from installing bspatch to running it. Both using terminal and not, preferably.
MCNostalgia Tool Changelogs: