There are a few ways you can approach this.
Mi Yodeya, Law, and Health all have "professional disclaimer" notices, which are placed on the right above the community bulletin. You can see Mi Yodeya's last request for more background on ours. For all three of these sites, the function of the notice is to deflect requests for personalized professional advice. The absence of such a notice was making some professionals -- the experts we want to see answering questions on our sites -- reluctant to participate. Your situation sounds different from that, so I don't know if SE would allow the use of this feature for your case. You would need to have your moderators request it from the CMs.
Another approach is the "events" section of the community bulletin. On some sites you'll see time-based notices there. (Mi Yodeya uses these heavily.) A moderator can create an event, specifying short text (a couple lines) and a link on your site. (It can't be an arbitrary link.) The event has a start date and an end date and is published during that interval. I've never experimented with long-running events, so I don't know if you could set the end date to 2038 or something and forget about it. Do note that if you use other events too, this one might get pushed off of the list at times; the space in the box is limited.
From the comments, it sounds like you want this so you can deflect a certain class of questions. If that's the case, you can ask for modifications to the "how to ask" help that comes up when somebody starts to ask a question. I haven't done this and don't know all the details of what's possible, but CMs can do this if your moderators make a request. Relatedly, if there is a specific tag that is causing you problems, you can add a notice to its tag wiki excerpt (see halacha on Mi Yodeya as an example).
Another answer suggested using featured meta posts. The problem there is that featured status expires, so the community would have to monitor it ("oops, expired again!") and moderators would have to keep re-applying it. Something that doesn't require monthly moderator action would be better.