Darkle I believe the posts end up in "limbo". The posts are flagged with the "private API" of Flarum, which means:
- When the extension is disabled, the posts become invisible to everyone but remain in the database.
- If you purge the extension, the posts become dissociated from the extension and will remain in the database but never appear again for users even if the extension is re-installed.
If an extension edits the posts in any way, they might revert back to public messages though. It should generally not be possible to edit a post flagged by the private API. But if an extension had for example a CRON job that loops through all the posts in the database with no regard for the private API visibility scope, they could accidentally "touch" the post, cause Flarum to re-evaluate the private API state and since no extension is there to request the private state anymore it might revert to public state.
EDIT: another situation where a post might revert to public is if multiple extension had flagged the post as private. For example if one post was still under approval as well as shadow hidden, once this extension is disabled the post will remain visible according to the Approval extension rules and if it gets approved it will become a normal public post again.
If you want to delete all the posts permanently, you can perform an SQL query manually for all posts where
shadow_hidden_at is not NULL before the purge.