Last week, Ingrid Tieken-Boon van Ostade published a post on Simon Heffer’s discussion of into on this blog. In his discussion of into in Strictly English, Heffer mentions a closely related usage item, the use of on to versus onto, of which he says the following…
There is no such problem in distinguishing when a writer or speaker should use onto and on to, because onto does not exist.
However, this ‘non-existing’ item is discussed in as many as 34 usage guides in the HUGE database, and in most of them onto is accepted (sometimes grudgingly) since the early twentieth century. The OED gives citations for onto dating from the early 18th century onwards.
1715 Duxbury (Mass.) Rec. (1893) 105 [A] place gutted away by the rain down onto Mr. Wiswells land.
And although it appears to be virtually non-existent in American English usage, COHA shows steadily increasing usage of onto in the twentieth century.
use of onto in the Corpus of Historical American English, 1900-2000
Many (most?) online dictionaries (for example Merriam-Webster, Oxford, Cambridge, and Collins) accept onto as a word, with a distinct use that is separate from on to. But this is not new. The use of onto was approved of by Henry Fowler, who gave examples in which the ‘non-existing’ onto was allowed, but not on to in his Dictionary of Modern English Usage (1926)
Occasions for on or to or onto, but on no account on to: Climbed up on(to) the roof; Was invited (on)to the platform; It struggles (on)to its legs again; They fell 300 ft on(to) a glacier.
Later approvers of onto also include otherwise conservative usage guide writers as Patricia O’Conner in Woe is I (1996) and Bernard Lamb of the Queen’s English Society in The Queen’s English and how to use it (2010).
So the information in Strictly English on this usage problem is not helpful. Heffer’s denial of a use of onto not only fails to take note of the historicity of this usage item, but it also fails to acknowledge a useful distinction. When it comes to onto as a usage problem, it seems we had it sorted out quite some time ago. So let’s go …
Bridging the Unbridgeable 
It was a shock to me, too, to hear that ‘onto’ doesn’t exist, particularly given that the word has also made its way into my Webster’s New World Dictionary (which I use to check for differences between UK and US usage).
Very interesting indeed! It reminds me of Caroline Taggart, who writes in Her Ladyship’s Guide to the Queen’s English that worser “is not a word in modern English” (p. 30). Of course it is, you only need to Google n-gram the word to see that it is. The word even has an entry in the OED, which reads that it has been around since the late 15th century (last quotation from the 19th century, but the entry hasn’t been updated yet). What is more, Victorina González-Díaz wrote an article about it in 2008. A COCA search produced two examples from 2011. “Doesn’t exist” and “is not a word” are just common arguments advanced by usage guide writers like Heffer or Taggart and perhaps others as well to express their disapproval of particular words or expressions.
He doesn’t say that onto doesn’t exist once, but three times! On p. 119, on p. 157 and again on p. 159. Does he feel he is not being heard or what?