We do get it. Check out this link to the current regulations. Note the date at the start. It’s regulations 2017 and current from date April 16 2018, so cant get any more current than this. Then go to schedule 2, division 10 rule 94. At no point does it say brake lights can not flash. It even still states for indicators that they can be red. There is absolutely nothing in what is the most current regulations that state brake lights can’t flash. You can even track the amendment history and there is no amendment for the rule you are quoting as being accurate. With a history in the road safety arena for 17 years, I know that all amandaments to regulations, Acts, legislation etc must be publicly available, and are made so via the internet. Often the only things not made publicly available are internal RMS policy, it as you have stated this is an amendment in the regs and therefore not an internal policy.
In any area where the public are required to comply with the legislation and regulations, such as licensing, road rules and registration, the information that they need to comply with must be made publicly available to them. Whether or not the public is notified of the change is a different story, as it sometimes the case with road rule changes. They might be advertised in the paper or something, but each licence holder is not contacted to tell them of the change. However the information is easily accessible for the general public to read. This is also the case with registration requirements. The fact that nobody else is able to find this rule that you are quoting, and it doesn’t even appear in the most up to date regs, would suggest that there has been a massive misunderstanding from some vehicle inspectors. I know of several blue and pink slip inspectors that have bend asked about this over the last week, and not one of them believes this to be a rule, despite regular inspections of classic cars.
https://www.legislation.nsw.gov.au/#/view/regulation/2017/451/sch2