libtorrent has repeatedly ignored requests for sequential download because of the silly spec. There's a patch out there that quite easily enables it. It's a bit absurd to me because there's a very good use case for sequential download and it's not clear there are any actual side effects.
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Doesn't sequential downloading degrade the swarm performance?
Maybe, but probably less relevant for seeds that are not new. Also if people are streaming via torrents it's a net benefit if all peers have early bits not just random data. In short I do not have empirical evicidence but I do believe the decision to do so should not be made by a library and I also believe the intial assumption that random is better holds true today.
Yes, but only 2nd order...
If all users downloaded in order, then users downloading the file cannot exchange chunks with other downloaders.
That "exchange" is necessary to encourage users not to limit their upload speeds.
In today's world where the vast majority of users just use the default settings for their torrent client, it probably doesn't matter much.
Libtorrent-rasterbar has supported sequential downloading for a long time. Are you sure you're not thinking of the other libtorrent (libtorrent-rakshasa)?
See my comment https://news.ycombinator.com/item?id=23580765 if you want an alternative.