This can be a murky subject. The fact that material has been removed from its electronic home might indicate that its original creator no longer supports it -- that the information in question is no longer valid. However, many documents on the Web are lost simply because a domain expires or because site reorganization breaks a link.
Unless the original author posts an explanation or retraction, it is usually not possible to know whether an item was taken down purposefully or has simply become unfindable through neglect or error.
If you have kept a copy, you can cite your source in roughly the same way you would cite a personal communication or an item from a personal collection. This citation need only occur in-text or as a footnote/endnote, not in the works cited or bibliography section.
Jordan complained in his since-deleted Instagram post, "The fact that hundreds of ppl are calling my comment homophobic ... you are so off the mark" (archived material, 2017).
Some digital material is created with an intentionally short life, intended to be viewable only briefly. Snapchat is perhaps the most well-known example; Instagram stories work in much the same way. Other digital material is in a state of constant flux. Search results, including the number and content of hits, and indexes of digital publications also change day to day, or even hour to hour.
For this kind of material, therefore, it is not only a good idea but an imperative to keep a copy if you intend to use it as a source. As with deleted sources, if you maintain a copy, you can cite it as you would a personal communication or collection.