I would do three things:

1. write tests
2. do not explicitly require order of comments
3. if down migration is not present, trying to run it should fail

1. write tests

👍

2. do not explicitly require order of comments

👍

3. if down migration is not present, trying to run it should fail

What do you mean?

-- up migration
insert into ...

-- down migration
-- nothing to do

Do you think that should fail? Why?

It should keep current behavior. Where you can't run down migrations on SQL files.
The same as for normal migrations. You can write explicit down migrations, disabled it, or Terry to infer it from up migration. But you can't infer down migration from SQL string. It failed down migrations

I'm still a little bit confused (and my english skills lags 😅)

I totally understand that we can't infer down migrations from up migrations, this wasn't my point
But I dont think it should fail when there is no SQL found below the down migration-comment

I'm probably describing it wrong. It should fail if running down migrations and there is no down migration comment.

OK

I dont think so

-- up migration
insert into ...

should work, imo

edit:
so IMO i mean follwoing should all work

example 1

-- up migration
insert into ...

same as example 1

insert into ...

example 2

-- down migration
delete from ...

example 3

-- down migration
delete from ...

-- up migration
insert into ...

example 4

-- down migration
delete from ...

-- up migration