A website is using this HTML meta tags:
<meta name="revised" content="Monday, November 23, 2015, 2:31 pm" />
But I read on this answer that it is not usable with HTML5 as it's not defined in the specs.
Has it ever been defined in past specs? Or at least been used by some browser vendors?
As explained in the linked answer, it’s not valid to use the revised name in HTML5, because it’s not registered on https://wiki.whatwg.org/wiki/MetaExtensions (as of 2015-09-09).
(In HTML 4.01, it’s valid, because there is no registration required.)
The page http://w3bits.com/meta-tag-revised/ claims that this meta tag is (or at least was in 2013) used by search engines ("Google, Yahoo!, Bing etc."). But it doesn’t give any sources, so that claim doesn’t appear to be trustworthy. Their WordPress plugin Revised Meta Tag doesn’t give any sources either, they only refer to their own page.
Several weeks ago the same question was asked on Webmasters (Do web crawlers use the “revised” meta tag?), but no evidence was found that this meta tag was ever consumed by someone.