Ѣ is the symbol that was used in Russian and was removed after the October Revolution. The "Russian (Polyglot and Reactionary)" layout includes pre-revolutionary letters "Ѳ", "Ѵ", "І", but it does not include "Ѣ". If I understand correctly, the Russian (Polyglot and Reactionary) layout is KDE-specific and is not available in other desktop environments, which is why I am writing this report here.
I can't find evidence that this keyboard is KDE-specific. I'd recommend asking the XKB folks about it.
The layout does have “Ѣ”, see: https://xkeyboard-config.freedesktop.org/layouts/analyzer/#?model=ansi-alt-60&layout=ru%28prxn%29 It is accessible using Caps + Qwerty T. You can find further info on the layout at: https://gitlab.freedesktop.org/xkeyboard-config/xkeyboard-config/-/blob/1357c07d55b8d21dd14be5780ff8eb59faca6818/symbols/ru#L961. Please report layout issues there: https://gitlab.freedesktop.org/xkeyboard-config/xkeyboard-config/-/issues