Unsupported Macs are not relevant to this discussion, because they cannot be enrolled in the first place. The problem at hand is that an enrolled Mac will always look for beta downloads first and their build numbers usually supersede the ones of the release builds. This is why the Mac must be unenrolled. After that, it will only look in the release mirrors for updates, which is what the OP wants. Unsupported Macs cannot, without manual modifications of the mirror paths, download system updates.
It depends on what you mean by "enrolled".
A case in point.
As I mentioned in another thread somewhere, on two separate occasions, -- once near the beginning of the macOS Sierra beta program, and two days ago again when the GM was released -- Apple sent me an email invitation in which I was invited to download the Sierra beta, and then the Sierra GM. This was obviously because I had previously participated in beta programs, such as for El Capitan, for example.
I was of course surprised to receive these invitations, being as I knew that my machine was unsupported, based on online info.
Nevertheless, in both instances, I followed through by clicking on the link in the email message.
I was taken to the enrollment page for Sierra, and I was in fact given a redeem code by Apple's server.
So, I was, and still remain, enrolled, as far as I know, and I anticipate that with the next beta program, I will receive yet another automated invitation message from Apple.
However, as I said before, the problem occurred when I actually used the redeem code in the App Store app on each occasion. That is when their server informed that I could not download or install Sierra on my machine, because it is about six months too old.
Personally, I find this disheartening and frustrating. Apple needs to fix their email notification system so that the owners of unsupported machines who are already on record from previous beta programs are not sent email invitations for beta programs in which they cannot really participate. Why give them false hope like that?
BTW, the OP only said "I downloaded the gm version of siera last night on my imac", so the details he provided were rather ambiguous. He does not say whether he got it from Apple, or elsewhere. Neither does he mention the year or model of his iMac. So, in my view, my comments were valid.
If you don't understand why I am saying this, it is because I also downloaded the GM on my iMac. However, it was not directly from Apple, or on a supported iMac. If the OP would have clarified those two points from the start, then I would have saved my breath, and said nothing.
But getting back to the enrollment issue, I suspect that you personally don't consider one "officially" enrolled in the beta program, unless the beta downloads option appears in the App Store prefs pane, but that is not apparently how Apple views it, since they did issue me a redeem code twice.
