MCPBot is finally dead, and so goes with it every old version of ForgeGradle. Hope you're not maintaining any 1.7-1.12 mods anymore (or, god forbid, 1.6)! Because if you do, nobody will help you, and if you ask you will be banned from all official Forge channels. Fun fun fun. Game over.
...Or not. Someone asked ArchiveBot to rescue the MCPBot exports, and a WARC file containing a seemingly complete archive is available for download. And, these old ForgeGradle versions download mappings over plain HTTP, not HTTPS!
I maintain Ears, a mod available for too many versions of Minecraft, and it uses old versions of ForgeGradle to build the 1.6, 1.7, 1.8, 1.9-1.11, and 1.12 ports. Because reengineering these old versions of ForgeGradle is a huge goddamn waste of time.
Except, on November 17th 2021, at about 20:00 Eastern time, MCPBot died. I was attempting to release v1.4.1 and was met with the error shown in the title. So, I downloaded the WARC, popped it on my server, and started serving it. (I serve a copy direct from my domain instead of redirecting to archive.org as archive.org is very slow.)
If you too maintain retromods and are hit with this problem, add this to your /etc/hosts and watch your troubles disappear:
192.99.194.128 export.mcpbot.bspk.rs
Ideally, you would use it from my domain and over HTTPS, but this is a good quick-fix and I'm able to handle the load. (I run other services that are fairly popular.) The nature of this as pure static files also means this should never become burdensome to keep running.
That only resolves the issue if you have all the mappings cached already, which is likely true, but if you blow up your ~/.gradle or try to build on a new computer it won't work. A complete rehost allows bootstrapping.