When developing an app, is automatic update enabled for all releases? Or is it only valid when using phased release?
Keeping in mind that auto update is user controlled, and the user can choose if for all, or app/by/app, yes, auto update is for all releases of a given app, regardless if the dev has chosen phased release.
Under the dev's control, a phased release only targets selected users that have auto update enabled.
See the docs: