Almost all zones follow the guideline. Coaches manufactured earlier than 1993 are very rarely seen around now. Not to mention, many of the trains were running with Vacuum braked coaches before the 90's that have been largely put out of commission now.
The rule itself is not really a stringent 'rule' - it is just a general guideline. It is just a good time to do a quality assessment for the coach before putting it out of service.