Linux 5.15 Raises Its GCC Compiler Version Requirement
The Linux kernel to now supported building on kernels as far back as GCC 4.9 while now it has been bumped to GCC 5.1. AArch64 already required at least GCC 5.1 while this bump affects all other architectures.
The impact though should ultimately be minimal except for those trying to build still on very old enterprise Linux distributions... GCC 5.1 dates back to the 2015 GNU Compiler Collection release. So basically you'll need a compiler within the past six years to build the mainline Linux kernel moving forward.
By dropping pre-GCC5 compiler support, they are able to avoid some compiler warnings of GCC 4.9 and drop other workarounds specific to GCC4. In fact, dropping the pre GCC 5.1 support means around 350 lines of code removed and lowering the maintenance burden around verifying old compiler support moving forward.
Linux creator Linus Torvalds back in 2014 referred to GCC 4.9 as "pure and utter crap", so he is probably happy now to see this support removed from the kernel.
Beyond removing old compatibility remnants and dealing with compiler annoyances, this move to the GCC 5.1 minimum version does mean that the Linux kernel can potentially switch from using C89 (GNU89 dialect) to C11 (GNU11 dialect) for future code, but at this time that change hasn't been made yet for mainline.
More details in this merge phasing out the GCC pre-5.1 compiler support in Linux 5.15.