Assembled Extra: Congratulations To Danielle Corsetto On Ending Her Girls With Slingshots
Full announcement here; you don't need me to repeat chunks of that information just to fill out a post, but it sounds like an organic move based on where the strip is creatively. Early strips will reappear in color.
I'm always delighted when a creator ends a long-running project on their own terms. You should see a significant display of affection and real sadness from the feature's legion of fans, most likely starting in the comments section of that linked-to post. Girls With Slingshots started in 2004, and was a major work of the webcomic as classically-formated comic strip era.