Yes. November 14th, 2012 (previous codename was "linefeed"):
Re https://t.co/GbFbiofkWS pic.twitter.com/mhdkWk16o2— Stewart Butterfield (@stewart) September 27, 2016
So Slack stands for ‘Searchable Log of All Conversation & Knowledge‘.
Can’t help wondering where they’d be if they’d stuck with ‘Linefeed’ as their product name?!