It does in fact exist, especially not just in the form of Coptic and Arabic but also in the form of Ajami which’s almost any native African language written in the Ajami script and which’s quite possibly the dominant mode of African writing for centuries. Possibly more African demographics still use this kind of writing, but it’s best preserved and more prominent in places where Islam dominates. But it could be argued that it does exist outside of Muslim majority countries to some extent, especially whenever Arabic influence predominates in Africa.
Ajami documents are interesting in which if translated, we could get a glimpse of how languages outside of Coptic evolve. Any surviving document will give a glimpse of how the languages evolve and develop, alongside any knowledge of medicine and folklore. (I even think a printing press of sorts did exist in Africa, which might account for any high possibility of surviving copies of certain documents.) Not to mention European languages in Africa started popping up in the 19th century or a little earlier, which’s when much of Africa got colonised by European powers such as France, Germany, Italy and Britain (but only France and Britain were more successful).
Under their guidance, we see the emergence of African languages written in the Latin script such as Yoruba Bibles and newspapers for instance. But the fact that much of written African literature was done in the Arabic script attests to the influences of both Islam and Arabs have on African people. Likewise the literacy rate in Africa might be much higher if only Ajami were included, then again that would also mean that literacy in Africa might be around much longer and embedded in the cultures more deeply. More Ajami documents will be unearthed eventually, which gives more insight into what written African literature was like before European colonisation.