Support for installing UKIs via kernel-install was added to systemd-253, the relevant commits are: 

3d5f0bfe4e72 kernel-install: handle uki installs automatically
642617f43145 kernel-install: remove math slang from man page
c7314ee7e290 90-uki-copy.install: create $BOOT/EFI/Linux directory if needed
78088b8f4371 bootctl: tweak DOS header magic check
2d4260482cb8 bootctl: kernel-inspect: print os info
24a3b37f12a1 bootctl: add kernel-inspect to --help text
a05255981ba5 bootctl: add kernel-inspect command
53c368d71ba4 bootctl: add kernel-identity command

The functionality is needed for RHEL UKI (kernel-uki-virt) package as currently it uses a 'cp' stop-gap and this far from ideal. E.g. custom kernel install scripts from /etc/kernel/install.d/ are not being called.

For additional context: in Fedora, 'uki-direct' package was introduced to assist setting UEFI vairables (BootXXXX/BootNext/BootOrder) upon UKI updates. This package also relies on kernel-install being called for UKIs and so this systemd feature is a pre-requisite to get it backported into RHEL. Even more details on the approach are available here: https://fedoraproject.org/wiki/Changes/Unified_Kernel_Support_Phase_2