I think it is fair to say that a quest for efficiency has reduced true airspeeds for the "average" commercial airliner, in two obvious ways.
1) as referenced above by DavidReidUK's link, higher and higher jet bypass ratios, which can have a higher peak efficiency, but at a lower TAS, and
2) higher and higher cruise altitudes (Mach .88 is "slower" at FL420 than at FL320 - 505 kt vs. 515 kt, assuming otherwise constant conditions - but burns less fuel)
"Workhorse" 707s and 727s used to zip along at 490-510 kts TAS. Current A320 family members or the 737s are engineered with a different goal in mind.