See T341142 for details. The column was created with a default value in T341679, and the default value can be removed once r937199 reaches production.

ALTERs to run:

ALTER TABLE campaign_events ALTER COLUMN event_answers_status DROP DEFAULT;

1. Where to run those changes: These tables are all in the extension1 cluster. In particular, they can be found in the per-wiki databases of testwiki, test2wiki, and officewiki, as well as in the wikishared database. So it's a total of 4 places.
2. When to run those changes: After r937199 reaches production: in wmf.18, next week.
3. If the schema change is backwards compatible: Yes (all write queries write to this column explicitly)
4. If the schema change has been tested already on some of the test/beta wikis: Tested locally and in beta
5. if the data should be made available on the labs replicas and/or dumps: Maybe, but we don't replicate data from x1 anyway.