One of the best ways to “capture a web page as it appears now for use as a trusted citation in the future” is the Internet Archive Wayback machine. It’s an awesome handy tool. You’ll be glad you know how to use it.
Publications sometimes change their minds and “unpublish” pieces. It is often under government pressure and sometimes under public pressure from special interest groups. When a publication retracts an article or an opinion piece, it is usually because the management (editors and owners) realize that they have published something that on second thoughts they should not have — the equivalent of “oops, did I say that aloud?” The way to do a hurried retraction is to delete the piece from the website. This happens quite frequently in the twitter world. But the incriminating evidence remains if some people do a screen-capture of the relevant tweet.