Hard though it is, when a grown up son or daughter are living with a partner, the role of a parent changes. I think you are wise to be willing to step back and leave it, but agree that it would do no harm to make it known that you are still there for your daughter if needed. It is possible that your daughter's partner feels that it is his responsibility now, and one which is only too happy to fulfill. Perhaps he is afraid that he would look weak if he included you, or maybe it doesn't occur to him to do so because of the way his own mother is.
Personally, as a divorced parent of two boys, and a chronic silent (as in I tried not to show it!) worrier, it was such a relief when they left home to live with a partner or spouse. No more sleepless night if they were out in the car, waiting to hear them come in etc, it was nice to know that in the event of an emergency someone else, who loved and cared for them every bit as much as I did, would be the one to get the phone call. Of course it is not that straightforward, and when I have sensed problems with them I still worry, but it is the way of the world that parents be supplanted by a partner. When I was younger, still married, and my parents were alive, I am not sure that I would have wanted to share all my worries with my mum even though we had a very close bond, mostly because I would not have wanted to worry her. To me she became not so much a mother type figure, but more of a close and beloved friend on equal terms.
The only thing is, if your daughter is not actually married, I am not sure who, legally, would be the next of kin, who may need to give permission for certain forms of treatment etc, this might be an issue to be considered and perhaps discuss with them both.