Hey, get ready to be confused @frissdiegurke -- there's a third type of hook, rounding out the behaviours 😆
action hooks are, as @Mega describes, fired when something happens, and NodeBB doesn't care about the response.
filters are fired when plugins might change things. The majority of hooks in NodeBB are filters.
static hooks are fired when you need NodeBB to wait until your plugin is complete in order to do something. There is only one static hook available right now: static:app.load, which is fired on startup.
In this case, since the post is already slated to be deleted, but you want to do something before it is deleted... we may want to consider adding a new hook called static:post.purge.
If all you need to change is a template file, you can just edit the template file stored in build/public/templates in the static:app.load hook. Since templates are compiled on demand, this will work as long as ./nodebb build is run afterwards.