The only specific prohibition of "backing" is at s. 157 of the Highway Traffic Act:
157 (1) No driver of a vehicle shall back the vehicle upon the roadway or shoulder of any highway divided by a median strip1 on which the speed limit is in excess of 80 kilometres per hour ...
However, there is a careless driving prohibition:
130 (1) Every person is guilty of the offence of driving carelessly who drives a vehicle or street car on a highway without due care and attention or without reasonable consideration for other persons using the highway.
And there is the Criminal Code's dangerous driving offence:
320.13 (1) Everyone commits an offence who operates a conveyance in a manner that, having regard to all of the circumstances, is dangerous to the public.
It would be misleading to say that it is absolutely "legal" or "allowed" to "drive in reverse on a non-divided road that has a speed limit of more than 80 km/h". It just isn't prohibited by s. 157(1) of the Highway Traffic Act.
1. Note that "divided" in the question was just a paraphrase of "divided by a median strip." There are many sections of road in Ontario that are not divided in that sense, with speed limits above 80 km/h, including much of the trans-Canada highway.