Peak evenings were introduced when Oyster PAYG became eligible on National Rail trains, but
only for zones 1-6. Also off-peak cheap day returns were abolished on National Rail train journeys for zones 1-6.
For reasons unknown, TFL forgot to mention this when they heralded "a new dawn of travel" for Londoners after Oyster was extended to trains in Jan 2010.
The
FAQ on the Oyster PAYGO from the Southern Trains website explains...
Q: Is it true that there are evening peak restrictions as well as morning peak restrictions for Oyster?
Yes. Peak fares for Oyster pay as you go apply to journeys between 0630 and 0930, and between 1600 and 1900. This is different to the previous Off-Peak Day Return tickets which only had a morning restriction.
Q: Why are you not selling Off Peak Day Returns anymore?
When Oyster is introduced on National Rail services, Off-Peak Day Return tickets will no longer be available in the London zonal area, across all train operators. This is primarily because, under Oyster pay as you go, fares are calculated based on each single journey made rather than a return journey. Different fares are also applied depending on the time of day and zone passengers are travelling from and to.
It only applies to Londoners. Anyone outside of zone 6 can still get cheap day returns and evening peak prices do not apply to them.
Of course, TFL had previous form when they used the debut of the Oyster Card to change to a 2-tier time (peak/off peak) and 2-tier price (Oyster/cash) system on tube & buses and try to pass it off as a benefit!