The part of help center What kind of questions should I not ask here? (Wayback Machine) contains at the moment the following formulation: "But there are lots of other math Q&A sites where your question might fit right in, like math.stackexchange.com, Ask Dr. Math, Art of Problem Solving, Physics Forums, or NRICH." Exactly the same formulation is used in the corresponding FAQ post on meta.
This part of help center is relatively prominent. It is probably viewed quite a lot, since one of the close reason which is quite often used links to this page: "This question does not appear to be about research level mathematics within the scope defined in the help center."
This particular page in the help center is editable by moderators.1 The FAQ post on meta is editable by regular users (with sufficient reputation), but the intention is probably to have the same wording in both places.
As you can see, several of the links given there are outdated for some time. (By which I mean that they are no longer usable for asking questions.) In the FAQ for Ask Dr. Math you can see the following text: "We will no longer be accepting new questions for this service starting on October 31, 2017." And on the Ask NRICH site you can read that: "Please note that the AskNRICH boards below closed on 31st July 2014." So those links actually do not serve the purpose for which they were included on this page.
Proposal: I suggest to replace the text with some information that is more up-to-date. Moreover, to make this part of help center easier to maintain, I suggest that instead of listing the sites, the help center could link to a post on meta which contains some suitable sites. (In this way we achieve that even regular users can edit the list if some of the sites no longer works. And an additional advantage is that there is only one place which needs to be changed when there is a need for an update.)
One possibility would be to use the answer to this question: My question was closed on MO because it is not research level. Where should I ask instead? (Of course, I might be biased a bit, since I started that post.)
1See also: What parts of the help center can site moderators edit? and Are all terms clearly defined in Help fragments? If not, who can contribute?