This news was revealed through the ongoing Apple v. Epic court battle. Documents detailed Epic’s mid-2020 plans for the then upcoming year of 2021, revealing the game would fully release for mobile. Slide 91 though, does not directly state that Rocket League mobile is planned for a Q2 2021 release.
The new beta update will be a ‘next-generation client’ which will allow cross-play and cross-progress across all platforms, including mobile.
Rocket League is coming regardless this year and will not be at the expense of Rocket League: Sideswipe as ‘Rocket League rebuilt for touch controls’ which is likely to launch first, especially after it held a limited alpha in Australia and New Zealand this year.