For someone not following the project, what's the reason for deprecation and switch over to LLVM ?
As someone messing with LLVM a lot.. basically LLVM is where a ton of development effort is going. GCC is a dead end it seems like. LLVM is designed to be extended where as GCC.. well, usually doesn't really seem to have been designed.
LLVM is designed to be modularized and embedded in other binaries and toolchains, which makes it easier to build extensions and tooling around it.
In addition to having a mountain of legacy code, gcc's project steering is philosophically opposed to allowing non-free (as in GPL) plugins [https://lwn.net/Articles/582242/]. Because of this, developers looking for the most flexible toolchain seem to be backing slowly toward the exist regarding gcc, as the Clang / LLVM steering team has made clear that they don't see similar incompatibilities between their tool and the way developers choose to use it.
There is a project going on to break the gcc down into components [https://gcc.gnu.org/wiki/rearch], so time will tell what comes of these two toolchains.