An urban myth is something that doesn't really happen or is grossly exaggerated.
The British Bankers' Association Banking Code defines a cheque drawn over 6 months previously as "out-of-date" and members of the BBA must have a specific stated policy on how they deal with such an out-of date cheque, be it rejection or otherwise.
http://www.bba.org.uk/bba/jsp/polopoly.jsp?d=3 47&a=13086
The banks operating such a policy of rejection (most of the industry) will not ask permission of the issuer to honour said out-of-date cheque; they will return it to the person presenting it and suggest that person request a new cheque be issued.
None of this equates to an urban myth.
Furthermore, in the case of dividend cheques, it is the norm for them to have a specified validity such as 6 months, 1 year or whatever it may be.