From what I recalled at that time — It couldn’t be a migration. Migrations ideally don’t touch application code (because code can drift from when the migration is intended and error out).
The PR also indicates the reason:
This PR also includes a rake task (
doc_categories.rake) for rebuilding sidebar sections and links from active doc index topics – this is not exactly recommended to do in a migration as it would involve either using the parser or nokogiri (which implementations can drift and cause an invalid migration).