Surely it is an oversight but it appears several places.
Just downloaded what appears to be the most recent Mega328P data sheet from Microchip. It is a bit strange because it refers ONLY to Mega328P with no reference to M88, or M168. It has a document number of 7810D-AVR-01/15. NB: Errata notes the removal of M88 and M168 from the spec sheet.
In the Features list on Page 2, it lists ONLY 32 pin TQFP and 32 pin QFN/MLF
In the Ordering Information, it lists ONLY TQFP and QFN.
Yet, Mouser says that it has 15,376 of the PU package (28 pin DIP) available to ship.
Come on Microchip! Please get your story straight!
More importantly: It Matters! DIP users will come to the spec sheet to find the pin-out Without a 28 pin pin-out, the users will need to seek other, and possibly questionable "information".