Hi,
I'm not part of the Debian Qt/KDE Team, but as far as I know kirigami will be updated as part of the KDE frameworks, which probably means some work to rebuild applications and patch those that have trouble building against the newer version, which is probably the reason why Debian doesn't package every version of KF5 but sometimes skips versions. I suspect that the KDE frameworks will be updated after the Qt framework was updated to 5.12, but feel free to correct me in case that's incorrect.
In case you need to support Debian stable with your application, you will likely need to backport to an older kirigami version, since it's not certain that kirigami will be backported to buster or backports can be used in the environment you are targeting.
I hope this helps,
Jonah