Currently, ALTER COLUMN TYPE is blocked for any column with a pg_publication_rel dependency, even when the column is only in a publication's column list and not referenced in a row filter. This is because both column lists and row filters create identical entries in pg_depend. The code that raises this error was added in commit 5f4a1a0a [1]
ALTER COLUMN TYPE should have no adverse effects on column lists, so it makes sense to unblock that case. This is done by looking up the corresponding pg_publication_rel entry and fetching prqual (containing the serialized row filter expression). In case prqual is NULL, the pg_depend entry corresponds only to a column list and ALTER COLUMN TYPE is therefore safe to apply.
There is, however, an edge case when a publication contains both column lists and row filters, and the column being ALTERed is only in the former and not in the latter. In this case, we need to actually parse prqual and check if the column in question is present in it.
I've attached a patch that does both these checks, along with regression tests.
