As called out a few days ago, the Ubuntu Mainline Kernel PPA had stopped providing new kernel builds . In mid-October new daily/development and stable builds stopped appearing. For a few days the PPA was also 404'ing. Quite frustrating for those enjoying this feature as an easy way to fetch the very latest Ubuntu mainline kernel binaries in a manner that's easy for all and helping with reproducible testing, etc.
The good news is that the service has been restored and is back providing daily kernel builds as well as of Linux 6.6 stable and the Linux 6.7-rc1 kernels. This month of downtime has apparently been due to some infrastructure changes/issues.
In any event those wanting the freshest Linux Git mainline kernel builds for Ubuntu systems can find the kernel images at kernel.ubuntu.com . Kudos to Canonical for getting this great feature restored.