Implementation of the lock began in mid 2001 shortly after Saf-T-Hammer's acquisition of Smith and Wesson. Like most transitions it occurred slowly as old inventory was continued to be used to assemble firearms so the actual date of change for any particular Model isn't known to the public
While it does appear that we hear more complaints about Smith and Wesson quality today than we did f5 or 10 years ago. Smith and Wesson's production has also gone up dramatically. Ten or 15 years ago they were producing about 1,000,000 firearms per year or 2000 firearms per day. Across the last five years most of the numbers came in right at the 2,000,000 firearm mark that means producing 4000 firearms per day.
I am not really certain by what percentage quality complaints increased over the years but even if quality stayed exactly the same producing twice as many firearms would create twice as many complaints. And of course the Internet is famous for circulating bad news very very fast
Personally, I have had no more or less percentage of problems with Smith and Wesson's bought in the last three years in comparison to Smith and Wesson's bought 20 years ago. And I have bought quite a few Smith and Wessons over that time.