One of the changes for Angular CLI 11.0 is deprecation of extractCss configuration property.
Getting error “extractCss is deprecated” comes because it has default value as true now. extractCss can be safely removed from angular.json or during the upgrade schematics should do it for you. This means that we don’t use styles.js anymore during the application build. The migration schematic should remove unused properties or replace them with the new alternative once you do the upgrade.
https://angular.io/cli/build
–extract-css
Deprecated: Deprecated since version 11.0. No longer required to
disable CSS extraction for HMR. Extract CSS from global styles into
‘.css’ files instead of ‘.js’.