It is now confirmed that the Canon EOS Ra astrophotography full frame mirrorless camera will be announced next. And the Canon EOS Ra mirrorless camera is already appeared on an internal Canon roadmap for 2020. Canon also has mentioned the Canon EOS Ra in an EOS R leaflet on one of their official sites. You can download the leaflet here.
The specifications are still unknown at the time of writing, stay tuned!
Canon EOS Ra leaflet states:
The EOS Ra is a version of the EOS R designed for astrophotography. This camera has approximately four times the transmittance of hydrogen-alpha light (656 nm) as the EOS R. Photographs of subjects that reflect a lot of infrared light will therefore appear redder than they actually are. Also, as it may not be possible to obtain an appropriate color balance or uneven colors may result, shooting normal subjects with this camera is not recommended
Camera operations are essentially the same as for the EOS R. Refer to the included EOS R Getting Started Guide (booklet). Additionally, you can download the EOS R Advanced User Guide (PDF file) from the Canon website.