@_ohcoco_ @Voline Private posts are only private to a point, but outside of their recipients and the instance's admins (present and future), they shouldn't be searchable.

That being said, public posts ARE searchable by words (and not only by hashtags) as long as they show on a search engine referenced instance. Then, "site:<instance.name> <word>" works pretty well. Just because Mastodon itself doesn't allow to search by (non-hashtag) words, doesn't mean it can't be done. Be careful.