According to Canon Rumors, Sigma is currently working on an RF-mount lens roadmap, which is expected to be announced sometime in early 2020. Apparently, several distributors have received a bit of information about Sigma’s plans, but Canon Rumors’ source didn’t want to give additional details.
Canon Rumors believe that Sigma could make the announcement ahead of CP+, in February of 2020. Considering Sigma is already releasing mirrorless lenses for Sony E-mount and that the sales numbers of Canon mirrorless cameras like the EOS R and EOS RP continue to increase, it makes sense for Sigma to enter this market early. This would also be a great thing for customers, as we’d have additional lens options, which would lead to cheaper lenses for this mount.
Let’s wait until February and see what happens, but if Sigma announces RF-mount lens roadmap, there’s a big chance they are planning one for Nikon’s Z-mount as well.