I have an update from the SDK team!
They explained that in general, they will follow the LTS (long-term-support) schedule for each framework. Since Angular 9’s LTS ends Aug 6 of this year, then any development on the auth0-angular SDK after that date will not take Angular 9 into consideration. However, you can always use the current version of the Auth0 Angular SDK which supports Angular 9 until you upgrade to the next version.