Any discussion of hyphenation should bear in my what Sidney Greenbaum a committed descriptivist, it should be said) has to say in his Oxford English Grammar (11.33 - Hyphens in Compounds).
The main function of the hyphen is to link words that form a compound word. Compounds may be 'open', written in separate words (eg washing machine), 'hyphenated', linked by a hyphen (e.g. tax-free) or 'solid', written in one word (e.g. handkerchief). Also to be considered are hyphens that attach some prefixes to an existing word to form a new word (e.g. ex-husband). American English tends to use fewer hyphens than British English, but British English practice is increasingly following American practice in this respect.
Notwithstanding this caution he is clear about number compounds. He says simply that
Number compounds are hyphenated (11.33.6)
He gives, among the examples of this,
A 70-mile-an-hour speed limit.
On this parallel, A 20-mph threshold is appropriately hyphenated.
However, as Edwin Ashworth explained in his comment, above, the online evidence shows that the unhyphenated 20 mph threshold is now much commoner than the hyphenated version. This does not necessarily entail that the hyphenated version has become incorrect. It is becoming (or has become) non-standard.