these answers violate the expectation that answers should be given
"on-site", not just as an external link, as the real steps for the
answer are in the curl link.
No, they do not violate any expectation. There are disclaimers in place that content on Stack Exchange network sites doesn't guarantee validity, but there is no explicit statement that answers should be given completely on site, in fact very often it's not possible provide full steps on site.
For user protection, one could leave a comment or download code and see its contents to ensure nothing violates OP's security. It is often not possible to provide contents of the linked code due to its length or the fact that the linked binary may be compiled.
Some well known sites such as Github or Github gists can be easily opened and checked.
The concept of Buyer Beware holds true throughout the internet, and the internet has been around for quite a while now, so if OP doesn't know any better than to not check what is being provided by the link in the answer - that's their choice to some extent. As Martijn Piters said:
Our questions and answers use an open license. Anyone gets to have
access to that knowledge, including people with ill intent.
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these answers become invalid when/if the link changes
Stack Exchange, once again, is not responsible for the content of those links or their validity. It's up to the community to push the poster and say: "Hey, update your stuff" or edit the post by yourself.
From a security perspective:
This is not a practice that should be recommended to users, most
either require sudo, or eventually call sudo, thus giving the keys to
the kingdom to a random URL
Otherwise, there is no reason to criticize post that has valid steps listed just because it uses
curl are no different from downloading items by yourself, with only difference that content is being passed to the shell or whatever else. Again, I cannot stress this enough: Buyer beware.