Back in January I changed the minimum toolchain version required to build clang and LLVM: D57264. Since then we've release LLVM 8, following our process it's therefore now a good time to remove the soft-error and officially deprecate older toolchains. I tried this out last Tursday night to see if any bots complained, and I saw no complaints. I also manually audited bots and didn't see any bot that should break, but their toolchain information is unreliable and some bots are offline.
Once this patch stick we'll move to C++14 as we've already agreed.