Sorry for the delay. My research on camels in warfare is still ongoing, but I’ll post what I found so I don’t forget.
Due to the difficulty in finding studies dedicated exclusively to camel warfare, I privately requested sources from the user Hand_Me_Down_Genes from the link posted by @bigbossbro08, and he recommended the book Corpus of Early Arabic Sources for West African History, which luckily (since it doesn’t have a digital version) I had bought a year ago but hadn’t read yet. This user is a military historian and almost all of his comments are very interesting.
While waiting for his answer, I found this article (PDF) Was There a “Bedouinization of Arabia”? which summarizes well the current evidence for the use of camels in warfare. The relevant part to us is from the subtitle “Dostal, the shadad, and Camels in Warfare” onwards, although the previous section on saddles is also useful. It’s a simple article to understand, and it discusses the three examples of battles already mentioned in this thread. The author’s conclusion is that camels were generally used only as transport to and from battle, and the fastest way over uneven terrain.
That said, the author doesn’t discuss cases beyond the Middle East, his area of expertise, which is sad because I found two contrary cases in sub-Saharan Africa that, as far as I know, are not studied by anyone.
The first case concerns the Berbers of North Africa. The screenshots are from Corpus:
Al-Bakri on the veiled Almoravids
Ibn Khaldun on the veiled Berbers in general: Lantunas, Sanhaja, Gudala etc.
There are other mentions of camel use in war, but these are the clearest. It is also interesting that those Berbers in the passage about Bornu are Tuareg, who are descended from these veiled people, making it more likely that they fought on camels.
I found the second example by chance when I researched the term “nujub/najib” for camels. These are passages talking about the Beja people of northeast Africa (Egypt/Nubia/Ethiopia). I found the first one and others on the wonderful website medievalnubia.info:
The Beja […] ride camels and fight in a sitting position as if they were mounted on horseback: they throw their javelins without missing the target. — al-Ya’qubi, 9th century.
Other passages from MedNub mention Beja camel riders armed with shields and pikes/spears/javelins/swords, but this one is the clearest. If you guys want to take a look, just let me know and I’ll list all the authors I found.
This is from Ibn Selim el-Aswani (10th century), available here or on MedNub cited by al-Maqrizi.
Perhaps these examples are the exception to the rule, since they refer to nomads. But given that (1) both served as mercenaries and regulars in various Afro-Asian sultanates and (2) the aoe2 camel rider is a regional unit (on a dromedary, specifically), I find its representation in the game valid.