This isn't likely to stop Reddit themselves from monetizing the data for AI training purposes. Deletion is typically "logical" in these types of systems, meaning that it's "marked as deleted" but not actually deleted.

What it does affect is the ability for others to see the posts, which might be companies accessing the API for AI training purposes. At this point, we don't know whether this is a meaningful path that Reddit wants to go down. If it is, they could allow the API to return deleted posts and comments (theoretically).