Stroke rules aren't the only technical parts of swimming that change over time--sometimes turn rules change as well. The biggest turn change I can think of came while I was team rep, so I consider it a relatively recent development.
Back in the Old Days, backstrokers had to touch every wall while on their backs. We could do flip turns, but only after we touched the wall on our backs. In 1991 that changed--and the new turn caused a bit of a flutter on deck that first summer.
Backstrokers could now turn to their stomachs when coming into the wall before they touched and execute what was basically a freestyle flip. They still needed to be on their backs at the finish of the race and, unlike in high school swimming, at the transition turn from backstroke to breaststroke in the IM.
Initially, we followed--as we usually do--the USA Swimming rule exactly. However, over time we decided this wasn't working for MCSL. In USA Swimming, the backstroke turn must be a "continuous" turning action--the one pull (single arm or simultaneous double arm) must be part of the turn. If swimmers roll over on their stomachs too far from the wall for one pull to get them close enough to flip, they're out of luck.
The MCSL season is short. Many children are new to swimming and, perhaps just as importantly, many stroke and turn judges officiate at just a couple of meets a year. Judging whether a turn is "continuous" or not is sometimes a very sophisticated call and, frankly, we were getting some questionable disqualifications. We were also hearing many stroke and turn judges weren't comfortable judging the backstroke turn.
In 2002, to make the turn easier for both our stroke and turn judges and our swimmers, the League changed the rule to delete the need for continuity. Now if swimmers turn over too soon, they can still only take one pull, but they can glide or just kick into the wall--and the stroke and turn judges don't have to have philosophical discussions of what "continuous" means.
Swimmers still need to finish the race on their backs--and touch on their backs at the back to breast transition turn in the IM.