There is a lot of [[ https://www.mediawiki.org/wiki/Talk:User_account_types | confusion ]] as to what a 'registered' user is, now that temp users are a thing.
MediaWiki says that a registered user is any user with a user_id. However, colloquially, a registered user is a non-anonymous user. Temp users are a bit of a hybrid.
A documented decision needs to be agreed upon and made on what the actual definitions of these user types are, and how they will be codified in data fields. By removing the [[ https://github.com/wikimedia/schemas-event-primary/blob/master/jsonschema/fragment/mediawiki/state/entity/user/current.yaml#L44 | is_registered boolean ]] from the MW user entity event schema, we can avoid codifying the wrong decision.
`is_registered` will be true if user_id > 0, so users of this can still get the same behavior without this field.
Since we haven't reallly officallly finally announced the mediawiki.page_change.v1 stream, we can make this as a backwards incompatible change.
[] Stop producing [[ https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/extensions/EventBus/+/refs/heads/master/includes/Serializers/MediaWiki/UserEntitySerializer.php#87 | is_registered ]] in EventBus
[] Remove [[ https://github.com/wikimedia/schemas-event-primary/blob/master/jsonschema/fragment/mediawiki/state/entity/user/current.yaml#L44 | is_registered ]] field and rematerialize 1.0.0 schemas, including mediawiki/page/prediction_classification_change (T328899)
[] Drop existent `event.mediawiki_page_change_v1` and `event.mediawiki_page_outlink_topic_prediction_change` Hive tables so that field is removed. (We don't yet care about the data in these tables).