Turn 10 Studios mentioned that they already started testing Forza Motorsport since late 2019. Development probably begun halfway through 2019.That doesn't mean anything.
Forza Motorsport being a reboot could launch in late 2021 at the earliest. Plus, I imagine T10 must have started work on it somewhere in late 2019 or early 2020 after scrapping the whole project of FM8 I presume. If it's a GaaS title, than 2021 would make a lot of sense. If it's huge, than it would be 2022 but it won't be.
Therefore, 2021 would have to be the target date T10 will plan to hit.