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.
Thru is listed as a preposition, adverb, or adjective, but, despite its pedigree, it's still considered more appropriate in informal writing such as a text message, a tweet, or a road sign (such as "No thru street") than in a formal essay, professional writing, or a report.
Threw, Through, and Thru: How to Choose the Right Word - ThoughtCo
Thru is mostly used where the preposition through could be used (e.g. Monday thru Friday); it is less common as an adjective or adverb (I'm thru with the vacuuming).
Thru and through both have the same meanings and functions in a sentence, but they are spelled differently. They can act as a preposition, an adverb, and an adjective.
The main difference between ‘thru’ and ‘through’ is that ‘through’ is the standard spelling and should be used in formal writing, while ‘thru’ is more appropriate in informal writing and when referring to drive-throughs or drive-thrus.
Thru is an alternate spelling that should be used only in informal writing or when referring to drive-throughs/drive-thrus. As if all the confusion over the words through and threw wasn’t enough, modern English has piled on yet another homophone: thru.
Use through in formal writing and thru in informal situations or when referring to drive-thru.
It’s easy to get mixed up on when to use thru vs. through, but this guide can help you. Explore the correct ways to use each word with these examples.