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