My own two cents is that support should never be pushed off-site by anyone, no matter if they're an official representative or a Canonical employee.
Part of the function of Ask Ubuntu is to build a repository of knowledge for the future, as opposed to the solve-each-problem-as-it-comes-up method of working with a support desk. If the site permits users to move support off-site, the site could end up losing valuable content that may or may not be relevant to solve an issue. We can't assume that the OP or the answerer will go back and edit their respective post, so it's better to keep it on-site.
Imagine that the support call led to a discovery of some other problem that wasn't solved by the command in the answer. It would be valuable to document what the problem was, and how it was resolved so future people can find it in a search.
This compounds in cases of company-backed support, like System76. What if a user with a Dell laptop has the problem? They can't be expected to call System76's support number, and System76 is definitely not expected to offer support just because a computer is Ubuntu. We then have a problem where nobody except System76 users are getting the answer. Furthermore, System76 now has to take up the excess workload of answering the same question 𝑛 times. While I can't speak for them particularly, I can't imagine they'd be too happy about that.
Answers like this should be treated the same as any link-only answer. We can't guarantee the link/phone number/whatever will work for everyone.
Furthermore, as mentioned by @David in the comments, the answer might be an otherwise complete/valid answer with the phone number/support link added in at the end within the context of "if this doesn't work or you need help with this, contact us." In this case, the support link isn't adding anything useful to the site. Instructions should be presented clearly on-site (assuming general competence -- e.g. people know how to open a terminal and use
sudo). Similarly, we run into the same issues as additional steps required may be discovered, or we're otherwise losing valuable information because it didn't seem relevant at the time the answer was written.
This doesn't even begin to touch of the issues of people claiming to be System76 or Canonical support. Without a good way to verify these users' claims, we're stuck with the issue of any phone number or external link potentially resulting in a scam or other dangerous advice being given.