I was having a look at some stats today and had a look at the number of double centuries scored in various calendar years. I think this is perhaps the best indication I have seen so far of exactly how much flatter pitches have become since the end of the 90s.
From 1980 to 1989, 35 double centuries and no triples were scored.
From 1990 to 1998, 32 double centuries and 4 triples were scores.
From 1999 to today, 70 double centuries and 4 triples have been scored, in just 6 and a half years. Between 2002 and today, 46 doubles and 4 triples have been scored. The only year in double figures before 2002 was 1999, with 10 doubles in the year. In 2002 there were 12, in 2003 and 2004 there were 14 each, including two triple centuries last year, and by just mid-April this year we have already seen 6.
Part of this can be attributed to an increase in test cricket and the inclusion of Bangladesh, but not most of it. It's a truly remarkable indictment on the preperation of pitches today.