Why did Bionic kernel update to 4.15.0-24 get removed from the repos?
Solution 1:
See the bug report: https://bugs.launchpad.net/ubuntu/bionic/+source/linux/+bug/1779827
Once this Critical bug was reported with enough detail, Canonical engineers and community volunteer testers bisected the problem and had a patch within 36 hours.
Note that this bug was critical, and worthy of pulling because booting at GRUB from previous kernels wasn't a viable workaround. Some folks reporting had to reinstall to restore functionality.