The patch is about ALTER TABLE ADD COLUMN with non-null DEFAULT. Your step 1 does not have a default value so this patch will not affect the database behavior. Postgres will not do anything with atthasmissing or attmissingval on steps 2 and 5 because these are ALTER TABLE ALTER COLUMN DROP NOT NULL, which is out of scope of the discussed patch.
I see, thanks! I guess I was interpreting it as "this is the new way that default values work in Postgres", but really it's only specific schema changes that are affected.
