As per the article there has been a 15m underwater limit off each wall in butterfly, freestyle, and backstroke since 1998. Though the article doesn't state it there was a 10m limit in backstroke 1988, that was relaxed to 15m in 1991. The rule change was spurred after David Berkoff and Daichi Suzuki[1] battled in the 100 backstroke in the 88 olympics going mostly underwater on the first length[2]. Though FINA said it was for safety concerns, it was more likely that they wanted the strokes to be preserved and not have swimming boil down to a breath holding contest.

[1] http://swimswam.com/david-berkoff-daichi-suzukis-underwater-... [2]https://www.youtube.com/watch?v=ztDlHwqD7pA