The question of the earliest GCC compiler version to support for building the Linux kernel comes up periodically. The ideal would be for Linux to compile under all GCC versions, because you never know what kind of system someone is running. Maybe their company's security team has to approve all software upgrades for their highly sensitive devices, and GCC is low on that list. Maybe they need to save as much space as possible, and recent versions of GCC are too big. There are all sorts of reasons why someone might be stuck with old software. But, they may need the latest Linux kernel because it's the foundation of their entire product, so they're stuck trying to compile it with an old compiler.
However, Linux can't really support every single GCC version. Sometimes the GCC people and the kernel people have disagreed on the manner in which GCC should produce code. Sometimes this means that the kernel really doesn't compile well on a particular version of GCC. So, there are the occasional project wars emerging from those conflicts. The GCC people will say the compiler is doing the best thing possible, and the kernel people will say the compiler is messing up their code. Sometimes the GCC people change the behavior in a later release, but that still leaves a particular GCC version that makes bad Linux code.
So, the kernel people will decide programmatically to exclude a particular version of GCC from being used to compile the kernel. Any attempt to use that compiler on kernel code will produce an error.
But, sometimes the GCC people will add a new language feature that is so useful, the kernel will people decide to rely heavily on it in their source code. In that case, there may be a period of time where the kernel people maintain one branch of code for the newer, better compiler, and a separate, less-fast or more-complex branch of code for the older, worse compiler. In that case, the kernel people—or really Linus Torvalds—eventually may decide to stop supporting compilers older than a certain version, so they can rip out all those less-fast and more-complex branches of code.
For similar reasons, it's also just an easier maintenance task for the kernel folks to drop support for older compilers; so this is something they would always prefer to do, if possible.
But, it's a big decision, typically weighed against the estimated number of users that are unable to upgrade their compilers. Linus really does not want even one regular (that is, non-kernel-developer) user to be unable to build Linux because of this sort of decision. He's willing to let the kernel carry a lot of fairly dead and/or hard-to-maintain code in order to keep that from happening.