This just means you can't use the json1 extension with an old version of the library. But anything you write using it, will still be readable with an older vers

This just means you can't use the json1 extension with an old version of the library. But anything you write using it, will still be readable with an older version. Of course, if you do something like creating an index using a json1 expression, that will cause issues if you try to use the database with an older version.