If it be
Is it used? Yes. But mostly poetically and within ritualistic practices like church where I've heard it because of its persistence relevant to the Bible, drama class given Shakespeare's influence, and court proceedings.
How frequently? Certainly not on a construction site anecdotally. A quick google turns up 14 million hits of the construction:
This song title by the Lost Dogs
Lyrics by Leonard Cohen
If It Be Not I by a Mona Van Duyn
If it Be Not Fair in the New Yorker
References to the Bible such as here
I've personally heard it used in conversation, but usually from the erudite. Certainly not among children, English language-learners, and those who speak dialects related to their first-language. I know I have used it, but I tend towards the construction: "Be it that..." I certainly would code-switch in public as a general rule.
As for the subjunctive generally
Grammatical forms explicitly using the subjunctive are used at various rates among various language communities. In any spoken and written language, usage diverges considerably from subculture to subculture. If you were to ask me, (and yes I use the subjunctive even in spoken English as a native speaker), I would say that the use of the subjunctive probably still occurs at a higher rate among populations like English professors and philosophy majors than among poor, immigrant communities, and that the use of the subjunctive is used more than recognized because of the way it subtly alters syntax. From WP on English subjunctive:
In Modern English, the subjunctive form of a verb often looks identical to the indicative form, and thus subjunctives are not a very visible grammatical feature of English.
Often, two divergent practices can last side-by-side in languages for many hundreds of years as in the case of using 'they' as a third-person singular in place of 'he or she'. Be that as it may, certainly in the US, instruction in grammar declined in the '60s, my understanding being that there were concerns regarding its effects on marginalizing minority communities.
Those of us who speak in hypotheticals frequently, we academic sorts, find the subjunctive a very important tool because it makes obvious the conditional nature of the proposition. Is it almost dead? I don't believe that to be the case. Many people still use "to whom", for instance, or answer the phone "this is she". In fact, some expressions still require the subjunctive as there are no alternative formulations. (See link below for examples). I imagine subjunctive never completely die, and will continue on in some form as it has since the 'olden days'.