Comment # 4 on bug 1208069 from Takashi Iwai

Go to the download URL for the OBS repo, and download kernel-default.rpm,
kernel-default-extra.rpm and kernel-default-optional.rpm for the architecture
(x86_64):
  http://download.opensuse.org/repositories/Kernel:/SLE15-SP5/pool/

Then install them via zypper install (as root):
  % zypper in kernel-default-*.rpm

But, it'd be safer to increase the number of installable kernels beforehand.
Edit /etc/zypp/zypp.conf (as root), and modify the line defining
"multiversion.kernels" to add more entries, e.g.
  multiversion.kernels = latest,latest-1,latest-2,latest-3,running

This will keep up to 4 kernels without purge.