I always thought "in-demand", "on-call", when used in the adjective position before a noun should be hyphenated for sheer readability if nothing else, but just checking, I see these compounds used without the hyphen:
"Coding has become one of the most in demand professional skills...".
Thoughts, anyone?
As I noted in a comment posted beneath the question almost six years ago, it is extremely common for writers to omit the hyphen in situations such as "Cooling has become one of the most in demand professional skills. Their reasons for doing so range from accidental omission to a belief that the hyphen is unnecessary.
Because no one has attempted to answer this question after all these years, I thought it might be worthwhile to review the advice that various style guides give for handling preposition + noun compound modifiers when the modifier is positioned before the noun it modifies.
Bryan Garner, Garner's American Modern Usage, third edition (2009) addresses this issue in broad strokes in his entry for phrasal adjectives:
PHRASAL ADJECTIVES. A. General Rule. When a phrase functions as an adjective preceding the noun it modifies—an increasingly frequent phenomenon in 20th- and 21st-century English—the phrase should ordinarily be hyphenated. Hence the soup is burning hot becomes the burning-hot soup; the child is six years old becomes the six-year-old child. Most professional writers know this; most nonprofessionals don't.
[Relevant example involving a preposition + noun phrasal adjective:] for-profit firms
The Chicago Manual of Style, sixteenth edition (2010) provides a hyphenation guide (in tabular form) for compounds at 7.85. The relevant coverage appears part in the "phrases, adjectival" subsection of the "Compounds According to Parts of Speech" section:
[Relevant examples:] an over-the-counter drug | an up-to-date solution
[but] sold over the counter | his equipment was up to date
Chicago also notes (at 7.79) that preposition + noun compounds sometimes end up as closed-up single words:
7.79 The trend toward closed compounds. With frequent use, open or hyphenated compounds tend to become closed (on line to on-line to online).
This tendency is rather more complicated than Chicago seems to suggest. For example, I have seen very little movement in print publications from up-to-date to uptodate despite the term's incontestably frequent use and widespread familiarity. In any case, in the "on" subsection of the "Compounds Formed with Specific Terms" section of the 7.85 hyphenation guide, Chicago lists the following preferred forms:
online | onstage | ongoing | on-screen | on-site
The last two examples provide further support for hyphenating preposition + noun modifiers, although in this case they seem to go a bit farther and support the hyphenated form regardless of whether the compound term appears before or after (or independent of) a noun. Tangentially I note that onsite frequently appears as a single word with no hyphen these days, vindicating Chicago's "trend toward closing compounds" thesis in that case anyway.
Like Chicago, The Oxford Guide to Style (2002) doesn't distinguish between preposition + noun compound modifiers and other types of compound modifiers. Still, the rule it lays down at 5.10.1 ("Compound words") is broad and simple:
Hyphenate two or more modifiers preceding the noun when they form a unit modifying the noun:
[relevant example involving a unitary prepositional phrase:] the up-to-date records
Oxford then goes on to carve out exceptions to its simple rule for modifiers to deal with cases involving foreign phrases, adverbs ending in -ly, capitalized words, and scientific terms, and other special situations—but none of these bears directly on how to handle simple English preposition + noun compound modifiers.
Syntactically, as far as I can tell, "in-demand" and "on-call" are just simpler constructions of the same basic form as of "up-to-date"—they are compound modifiers consisting of preposition[s] + noun. The prevailing convention in both U.S. and British publishing seems to be to hyphenate this type of phrase when it appears in front of the noun it modifies. Whether Garner is correct that this rule is something that most professional writers know and most nonprofessionals don't, I can assure you that many professional writers do not consistently abide by the rule in the manuscripts that they submit for editing.
Sometimes the two words previously hyphenated become elided, so we get "tasks that are done every day" or "everyday tasks".
– Nicole May 25 '17 at 19:03