The fact that the SA CAA publish a clear warning on their AIP Ground Movement Chart for JNB, indicates that taxiing down Mike in error, was and is a known threat. Normally published 'Hot Spots' and warnings for taxying are normally derived from past errors that have been made. I would be very surprised if BA was the only operator that has had aircraft involved in entering Mike in error.
The SA CAA obviously felt that the threat was worthy of mention on the AIP taxi chart itself and not confined to the text portion of the aerodrome entry. If Navtech had done the same and published the warning on the chart itself, then perhaps there would have been more likelihood of the crew spotting this and perhaps even briefing it before hand?