Ubuntu's Great Mainline Kernel PPA Hasn't Been Working Since Mid-September

One of the very convenient features for Ubuntu Linux users who want to run the very newest upstream kernel releases or simply test a new kernel build for verifying bug fixes or functionality has been the Ubuntu Mainline Kernel PPA. It's been around for years, backed by Canonical, and very convenient for fetching pre-built AArch64 / ARM / POWER / RISC-V / x86_64 kernel binaries as Debian packages... With options of stable/test kernel versions as well as daily packages. Sadly, it's been broken once again and has been in that manner since mid-September...