It seems the 500 series motherboards are also scheduled for March 2021:
According to the roadmap, the gaming 500 series is due to launch in late March, which is more or less the same timeframe we mentioned in our article. Intel is allegedly planning to launch its Rocket Lake-S processor series in the 10th week of 2021 (so mid-March).A new motherboard should not be necessary for Rocket Lake-S, current LGA1200 motherboards should be compatible. The leak also indicates there will be no new HEDT platform for Intel until at least June 2021 (which is the cut-off date of the leaked roadmap).
On a related note, there's also a rumor that AMD isn't planning new chipsets for its Zen 3-based processors. Earlier rumors suggested there would be a new X670 high-end chipset. The new rumor suggests AMD will focus on the transition to DDR5 and Socket AM5 for Zen 4 next year.