Neither this post nor the linked site explain what XCancel is/was. Since I wasn't familiar with XCancel, an announcement that it's suspended is not particularly informative.
Apparently XCancel is an instance of Nitter. According to Wikipedia, "Nitter is designed to allow access to X without tracking, advertisements, or the need for an account. It only supports browsing, and cannot be used to sign in or interact with the X community."