Accents come under "linguistic peculiarity".
There was a wonderful documentary about Jacqueline Du Pre on BBC4 last night, with many interviews with classical music luminaries who know her, such as Itzhak Perlman, Dietrich Fischer-Dieskau, and Daniel Barenboim. There were also three black-and-white interviews, probably dating from the 60s, with William Pleeth, Sir John Barbirolli, and Jacqueline herself. All of the interviewees were highly-educated, upper-middle class types (upper-upper class in the case of the Duchess of Kent), and all sounded it, but what was very noticeable was how much more unashamedly posh Jacqueline, Pleeth and Barbirolli sounded in their archive interviews than the modern interviewees. This reduction in poshness has been ramarked by many commentators, but here was evidence. The voice-overs of old newsreels from the 40s and 50s are another example - no-one talks anything like that now. (I'm not sure that they did even then). It has been observed that the Queen's accent has changed over the decades, as well.