In case you missed it, the initials ICYMI stand for the first five words of this sentence. Individuals and outlets deploy it every few seconds to bring links to the attention of others who may not have seen them.
In an earlier time, the full verbal formulation meant that the recipient was supposed to see something of a certain degree of importance (a note, an email), and the sender was gently reminding you of its oversight.
Now, the very shortening of the phrase is a byproduct and acknowledgment of the velocity of information against which its attached link is racing. The implication is that you probably have not seen it, and it’s not necessary to your existence, but the sender would like to bring it to your attention anyway, please. It serves as a call of desperation as much as an announcement.