In some of the older lore, I believe it was plainly stated that there were Chaos Eldar factions. While that's been almost entirely dropped since then, we are given avenues of opportunity still.
In the 8th edition core rulebook, there is a vague chart of all things Aeldari, with each intersection labeled to indicate a subfaction. The legend names all of the subfactions but for one, which is ++REDACTED++.
While the image for that subfaction is rather . . . busy, and I couldn't tell you what it was depicting like at all, to the side of each subfaction's image are examples of sigils.
And the redacted subfaction's symbol is that of Slaanesh.