They are not serially incrementing but still sortable. Thus prevent index fragmentation issues observed with UUIDS. Are 8 bytes in length. So index size is smaller compared to UUIDs. So you get all benefits of serial IDs but they are not easily guessable thus preventing sequential access attacks.
What unimaginable scale
Today, a decade later, they're at 1.074B
I don't see how that's true. From reading the article you linked, you only need a valid shard ID (which you can extract from known IDs), the millisecond (which is guessable) and a 10-bit sequence (which you can easily brute-force).
(And that's completely fine if their security model doesn't require unguessable IDs.)
It will results in a very high number of 404s. These can be monitored and the origin IPs can be banned.
I’m suspecting you meant “easily guessable” in the human sense, not the cryptographic/security sense. My bad if I misunderstood you. Again, I’m not saying Instagram has any security problem, I’m just saying that this ID scheme in particular isn’t a security feature.