I discovered a signed version 5.600 available in biggest german speaking forum android-hilfe.de, in order to prevent unintentional update of K-9 - read here:
Wird K9 mail nicht mehr gepflegt? | Seite 7 – Android-Hilfe.de
There are steps described for switching to that version. If needed ou can try to translate via Google Translator or DeepL-Translator.