It looks like those links are working now, but they should not. Using the new API endpoint to demonstrate:
https://api.stackexchange.com/2.2/comments/{ids}?site=stackoverflow
This is not a URI. As mentioned previously, curly braces are unsafe characters, and are not valid in URIs (see RFC 3986). For the purposes of API documentation, and other similar uses, they should not be percent encoded, however, because they still aren't URIs - they don't identify a resource. When the embedded parameter expression is expanded, in this case by replacing it with a semicolon delimited list of ids, they do identify a resource. As such, it's a valid URI.
https://api.stackexchange.com/2.2/comments/1;2?site=stackoverflow
Using curly braces to indicate an embedded parameter in a URI-like descriptor - called a URI template - is defined by RFC 6570.