If the earlier leaked roadmap is true (might be as it matches with these three lenses as well), then these are the remaining lenses for the next two years.Just because 300 mm is not mentioned in this list does not mean that it will not be announced (Rumor site) or if not announced now, it will not be announced soon afterwards.
Canon TS-R 14mm f/4L
Canon TS-R 24mm f/3.5L
Canon RF 10-24mm f/4L USM
Canon RF 14-35mm f/4L IS USM
Canon RF 18-45mm f/4-5.6 IS STM
Canon RF 100-400mm f/5.6-7.1 IS USM
Canon RF 35mm f/1.2L USM
Canon RF 135mm f/1.4L USM
Canon RF 24mm f/1.8 IS STM Macro
Canon RF 500mm f/4L IS USM
Canon RF 800mm f/5.6L IS USM
Canon RF 1200mm f/8L IS USM
So a 300mm lens would come later.