I'm sure most people here are familiar with the saying Ehrlich währt am längsten, which is roughly equivalent to "Honesty is the best Policy." (The literal, and alliterative, translatation is "Honest lasts longest.") But shouldn't it actually be Ehrlichkeit währt am längsten? The wonderful usage database at DWDS only turns up results for the sans-keit version, so it's definitely the version that is commonly used. But the reason I'm bringing this up is that Wiktionary claims that it's grammatically incorrect or outdated. So the first question is whether it's okay in German to turn a random adjective into a noun without adding the expected -heit or -keit. I'm pretty sure doing the equivalent in English would be unusual but not always actually wrong, and there's the English saying "Clever is as clever does." The second question is whether Ehrlich währt am längsten is considered outdated. Perhaps the English equivalent is a bit old-fashioned, but no more than that.
„Ehrlich währt am längsten“ is not outdated. It is a bit old-fashioned, but not uncommon in modern context.
It has, as far as I can tell, also not been replaced by a newer saying.
„Ehrlichkeit währt am längsten“ would be more grammatically correct, but is rarely ever used.
As mtwde said, „Ehrlich währt am längsten“ contains an ellipsis. Such stylistic devices are quite common with sayings, as they’ve been refined over the years.
To answer your questions: yes, it should be „Ehlichkeit“; no, it is not okay to use adjectives as noun without -keit or -heit (save maybe few odd cases and poetry); no, it is not considered outdated.
Ehrlich währt am längsten is a really old German saying. Ironically, it can also be found in Karl Leberecht Immermann's famous book "Münchhausen" (written in 1838/39), for example.
bei den folgenden redensarten kann man sich den inf. sein oder leben hinzu denken: ehrlich macht reich, aber langsam gehts her; ehrlich währt am längsten. [...] erklärt sich nach ehrlich adj. 6.
In short: There is some kind of ellipsis in this sentence and you may add a "sein" to it.
Ehrlich sein währt an längsten.
With this in mind "ehrlich" is more of an adjective than a noun and a literal translation would be
Being honest lasts longest.