You may have an Archive Rule set up for a table (ex: incident). When creating new fields, or enabling a plugin that includes additional fields (ex: Major Incident), will these fields be added to the Archive table (ex: ar_incident)?

The answer is yes. Any newly-created custom fields on a table, as well as plugin fields are added automatically. Using the example mentioned, after the plugin Major Incident is enabled, the [incident.business_impact] field will appear on the archive table [ar_incident.business_impact].

No intervention is required; the next time the archiver rule runs, the archived data will include the new/additional fields.

Docs:  https://docs.servicenow.com/bundle/newyork-platform-administration/page/administer/database-rotation/concept/c_ArchiveData.html