"Thru" vs. "through"

I was going to comment that this is General Reference, but actually looking for a reference didn't come up with much.

Merriam-Webster has this...

While never extremely common, tho and thru have a long history of occasional use as spelling variants of though and through. Their greatest popularity occurred in the late 19th and early 20th centuries, when their adoption was advocated by spelling reformers. Their current use occurs chiefly in informal writing (as in personal letters) and in some technical journals.

Wiktionary notes the following, which gives more details on the "technical" usage:

Thru is generally only noted as a preposition (e.g., Monday thru Friday), and seldom as an adjective or adverb (e.g., I'm thru with the vacuuming). Additionally, it is subject to being changed by those who believe it is wrong. It is less used in formal situations, except in cases where brevity is wanted such as roadway signs or date ranges.

It is often noted in professional drafting: "⅝-inch thru hole". Thus noted to save space and simplify annotation on a drawing for fabrication or construction.

This spelling is not used in British English. Also, this usage [that is, "Monday thru Friday"] of the word "through" is very rarely found in British English ("to", "'till" or "until" would be used instead).

So yes, thru is some sort of slang or jargon.

In Strunk & White, E.B. White's answer was:

Do not write nite for night, thru for through, pleez for please, unless you plan to introduce a complete system of simplified spelling and are prepared to take the consequences.

White quotes the original edition, in which Strunk said,

From time to time new forms, mostly simplifications, are introduced by innovators, and either win their place or die of neglect.

White adds, somewhat wistfully,

A word that has taken hold in our century is thruway ... it is a high-speed word for readers who are going sixty.... It is conceivable that because of our thruways, through will eventually become thru -- after many more thousands of miles of travel.

(The Elements of Style, 1979 edition, p. 74-75)