Rule changes in the last four years wouldn't be very exciting, so I'll list some that have happened "recently." This isn't a comprehensive list, but it's got most of the major ones, methinks:
- 1988: 10m rule for backstroke (modified to 15m in 1991). David Berkoff immediately comes to mind when you think about the 15m rule, because he became famous for what John Naber (at the time, an NBC commentator) dubbed the "Berkoff Blastoff." He set a world record in Seoul staying underwater for 35m off his start:
If you've got good technique, you're much faster underwater in a streamline position than you are on the surface of the water, and Berkoff's dominance led to a new FINA rule in 1988, stating that a swimmer's head must break the surface of the water by 10m in backstroke. This was expanded to 15m in 1991, and there the limit sits today.
- 1991: allowing backstroke flipturns. I guess I can use the above youtube video to demonstrate another point - before this mid-1991 rule, swimmers were required to touch the wall with one hand before turning in backstroke. In breaststroke and butterfly, you're required to touch with both hands at every turn, and freestyle doesn't require you to touch with anything more than your feet - but backstroke was the odd one out, requiring a (what is now considered) sort of strange combination of a touch and flipturn. Some swimmers put this turn to good use today in the IM on the backstroke-breaststroke transition. It's a true testament to the great backstrokers of the 80's/early 90's who went so fast despite having to touch the wall with their hand every turn - you can only wonder how much faster they would have been without that requirement.
- 1998: 15m rule for butterfly. I can't find definitively in which year the rule changed for freestyle as well, but I'm assuming it's coupled with the butterfly rule. In any case ... Somehow, it took ten years after instituting a backstroke underwater limit for butterfly swimmers to successfully exploit the lack of underwater limitations to the extent that it prompted FINA to change the rules. One of the most high profile, successful swimmers who benefitted from staying underwater a long time in butterfly before the rule switcheroo was Misty Hyman (and if you haven't watched her 200 fly gold medal performance from Sydney on youtube, go do it now!). Needless to say, she did just fine post-15m rule.
- 1999: no-recall false start rule. Prior to this rule, if someone false started, the whole field would be called back and the offending swimmer disqualified. This was pretty disruptive to the other swimmers. This new rule meant that if someone false started, they'd be allowed to complete the race and would be disqualified after finishing. Harsh. I think there were different rules about false starts earlier on, but I couldn't find anything definitive.
- 2005: breaststroke pullout modification. If you were an American swimming fan watching the Athens Olympics - specifically, Brendan Hansen's breaststroke races - you were probably cussing up a blue streak and throwing things at the TV. Why? Well, it was clear that Japan's Kosuke Kitajima, who won both breaststroke races, was doing dolphin kicks during his pullouts off of each wall. This was unambiguously illegal, but somehow the only high-profile near-disqualification of the games was USA's Aaron Peirsol in the 200 back for one of his turns. But I digress! Instead of DQ'ing Kitajima, FINA decided to make his behavior allowable. This meant that everyone was now allowed to do one dolphin kick off each wall. If you watch the breaststroke races now, you'll see that some swimmers sneak in one or two more than that. Sigh.
- 2009: THE SUITS. You had to have been hiding under a rock to have avoided hearing about the great suit controversy of 2009. Swimmers basically wore compressive wetsuits that made them float and not get tired. I exaggerate, but you get my point. After the total circus of 2009 that resulted in the utter decimation of all record books, FINA put an end to it by requiring "textile" swimsuits of certain lengths for men and women. You'll often hear about "textile bests" in the post-suit era, and what they're referring to is a non-super-suit world record.