Looks like the functionality is purely in a kmod, because when I enable that option I get exactly the same sized image built. Can you validate that finding and perhaps we can include it by default if the size is not increased?
Actually - the sysupgrade is the same size but initramfs grew a small amount:
$ ls -al bin/targets-default/ath79/generic/openwrt-23.05-snapshot-r23672-fd6831731b-ath79-generic-glinet_gl-ar750-squashfs-sysupgrade.bin
-rw-r--r-- 1 100999 100999 6423349 Dec 19 09:35 bin/targets-default/ath79/generic/openwrt-23.05-snapshot-r23672-fd6831731b-ath79-generic-glinet_gl-ar750-squashfs-sysupgrade.bin
$ ls -al bin/targets-vrf/ath79/generic/openwrt-23.05-snapshot-r23672-fd6831731b-ath79-generic-glinet_gl-ar750-squashfs-sysupgrade.bin
-rw-r--r-- 1 100999 100999 6423349 Dec 19 10:49 bin/targets-vrf/ath79/generic/openwrt-23.05-snapshot-r23672-fd6831731b-ath79-generic-glinet_gl-ar750-squashfs-sysupgrade.bin
$ ls -al bin/targets-default/ath79/generic/openwrt-23.05-snapshot-r23672-fd6831731b-ath79-generic-glinet_gl-ar750-initramfs-kernel.bin
-rw-r--r-- 1 100999 100999 5617238 Dec 19 09:35 bin/targets-default/ath79/generic/openwrt-23.05-snapshot-r23672-fd6831731b-ath79-generic-glinet_gl-ar750-initramfs-kernel.bin
$ ls -al bin/targets-vrf/ath79/generic/openwrt-23.05-snapshot-r23672-fd6831731b-ath79-generic-glinet_gl-ar750-initramfs-kernel.bin
-rw-r--r-- 1 100999 100999 5617320 Dec 19 10:49 bin/targets-vrf/ath79/generic/openwrt-23.05-snapshot-r23672-fd6831731b-ath79-generic-glinet_gl-ar750-initramfs-kernel.bin
Only 82 bytes larger tho, which seems negligible.