On August 8, 2017, Segregated Witness was officially locked in by the UASF client on the Bitcoin network. This milestone was reached when 100% of miners signaled support for SegWit between blocks 477,792 and 479,807. Thus, the SegWit upgrade was locked-in and would be fully activated in approximately two weeks at the start of the next difficulty adjustment period.
Learn more about SegWit in Jonathan Bier's book "The Blocksize War."