Archive of the MCP mappings formerly hosted by MCPBot.
In modern times they are also hosted on https://maven.minecraftforge.net/. However, since Forge themselves have proven largely disinterested in maintaining the effort which was once put into creating these mappings, and since their repositories are generally unreliable, creation of such archive would appear to be a neccessessity.
The versions.json is the indexing file directly copied from Forge's Maven and can be used by ForgeGradle 1.x/2.x as-is. It is currently used by my ForgeGradle fork. The real_versions.json is an updated indexing that obeys the same format and contains all mappings which are actually present in the repository. It can in theory be used for the same purpose, but doing so would require additional changes in ForgeGradle itself, as it assumes no empty snapshot/stable/snapshot_nodoc/stable_nodoc entries can exist, which is not the case with last modern mappings.
Not here. They were never published on export.mcpbot.bspk.rs or Forge's maven, and can only be obtained with MCP itself (the list of MCP versions along with download links can be found here). If you will - you could help this archival project by catalogizing and submitting those older mappings through a PR.