I've never met this nostrum. How do you avoid starting sentences with abbreviations like these:
Mr Davis went to work on the same train every day.
St Paul wrote many Epistles.
NATO tanks moved across the boarder.
With some, there'd be no occasion to ("etc", "e.g." "C" - to mean "Celsius").
When I was at OUP, there was a house rule that we should avoid starting a sentence with a numeral '[The year] 1968 was notable for the summer of love'. But that was never presented to me as any more than an internal rule - it's one that a lot of people observe, but I wouldn't regard its breach as in any sense 'wrong'.