what in the first sentence is a relative pronoun in a 'non-restrictive' clause. My gut feeling is which could take the place of what in the sentence. I am not quite sure of my knowledge of the difference between the two in the 'non-restrictive' clause.
Online environments vary widely in how easily you can save whatever happens there, what I call its recordability and preservability. Even though the design, activities, and membership of social media might change over time, the content of what people posted usually remains intact. Email, video, audio, and text messages can be saved. When perfect preservation is possible, time has been suspended. Whenever you want, you can go back to reexamine those events from the past. In other situations, permanency slips between our fingers, even challenging our reality testing about whether something existed at all, as when an email that we seem to remember receiving mysteriously disappears from our inbox. The slightest accidental tap of the finger can send an otherwise everlasting document into nothingness.