It's not secret at all....... It's literally discussed in the original post of this thread.
You mean this part?
Why can't I access private messages, chat or the search bar?
To do that, you need a certain amount of contributions - we can't tell you the exact number. It doesn't matter if those contributions are comments or threads, both count towards the threshold amount. Do not spam low-effort posts though unless that fits in with the nature of the respective thread (e.g. posts asking members to post in one word what they are feeling).
If you don't tell someone what the limit or boundary is, it is for all purposes invisible. If you know but say that you can't say what it is, that makes it a secret. I had to go looking through this thread to find the answers because it wasn't mentioned in the initial greeting.