alt Hacker NewsWell from the (incomplete, as turned out) info I gathered it turned out that it was never standardised to begin with (unlike meta description, for example), so it did not even have to be "deprecated". (Like it ever meant something in the real world…) I am definitely for any obsolete mischief, so it brings me joy hearing there is vernacular continuity!
(As for external search engines,FWIR, in its heyday some search engines briefly regarded it quite respectfully, but eventually due to SEO spam they either started to ignore it, or took it as a negative signal, allegedly. But as pointed out in sibling comment, it still survives in Yandex, what is quite surprising plot twist for me.)
It's recorded in the HTML5 spec under 4.2.5.1 Standard metadata names, which defines the correct format and parsing:
https://html.spec.whatwg.org/multipage/semantics.html#meta-k...
So, it's standardized and not deprecated.
It has this note though:
"Many search engines do not consider such keywords, because this feature has historically been used unreliably and even misleadingly as a way to spam search engine results in a way that is not helpful for users."
HTML 4.01 says the following:
"This specification does not define a set of legal meta data properties. The meaning of a property and the set of legal values for that property should be defined in a reference lexicon called a profile. For example, a profile designed to help search engines index documents might define properties such as "author", "copyright", "keywords", etc."
https://www.w3.org/TR/html4/struct/global.html#h-7.4.4