At least according to Wikipedia references, BitTorrent Inc's own client had mainline DHT support since version 4.2 (~2005). uTorrent seems to have added support for mainline DHT some time before the acquisition as well. There's a good torrentfreak post from the 10 year anniversary that covers the history pretty well: https://torrentfreak.com/bittorrents-dht-turns-10-years-old-...

I'm guessing that your "by default now" may actually be in regards to BitComet and the v0.60 client that upset some folk?

Besides the BEP44 implementation magila mentioned, BitTorrent Inc. also assists with the DHT by operating a public bootstrap server at router.[bittorrent|utorrent].com, and provides the source too: https://github.com/bittorrent/bootstrap-dht