This is simply not true! Changing window title fonts and font-sizes works perfectly from the settings menu as shown here. I haven’t yet found a context where I couldn’t change the font.
Kubuntu 20.10
KDE Plasma version 5.19.5
(works also perfectly fine under Garuda Linux with KDE Plasma Version 5.21.0)
Maybe you didn’t get an answer (From whom? The KDE Team? The Kubuntu Team?), simply because it might have been a bug in an older version which has been fixed, maybe already for a while.
That’s why we get system upgrades from time to time.