Admittedly, I haven’t been following RPS for a while now owing to a lack of time - from time to time I will simply check the Reviews category, and read posts that are shared by other users.
That said, my belief is originally all Supporter posts were supposed to go public at some point, but the time it took for that to happen would vary from post to post. That they all went public is because the Supporter program was mainly a way to reward those who were willing to support RPS explicitly and who, before its introduction, would set up something like a recurring Paypal payment entirely on their own (some, I think, still do that).
For that reason, too, supporter posts tended to cover more niche areas, and were generally shorter.
As usual keep in mind mine is not the official word of RPS, just one guy like everyone else here: I don’t hang out in the super exclusive RPS lounge, nor am privy to any special information. Just my impressions and my understandings, which may be flawed and/or become superseded.