I think the older post notices can be hidden from individual sites using a little CSS:
However, I think after a certain window of time these notices become relatively meaningless and serve only as extra noise, which I think applies to all sites. It would be useful if these notices could tidy themselves up after their usefulness had passed.
Maybe it would be a good idea to just have .post-notice.old: display: none; shipped in the default CSS so people can always override it if they want to. That makes the change very small and it leverages the existing .old functionality.