Arc System Works has yet to officially announce a release date for Kill la Kill – IF other than 2019 as a window, but it appears to have been revealed through a little press release slip up as July 25 in Japan.
The release date of July 25 was given as part of a press release for the upcoming tournament Toushinsai 2018-19. It was published by Famitsu, 4Gamer, and Gamer, but has since then been edited to state “2019 release” shortly after.
While it doesn’t confirm the release date, it appears very likely to be it. In any case, Toushinsai starts on March 23 so it shouldn’t be long until we get something official from Arc System Works.