Decades of tax information provides you a view

The subject is "Decades of Tax information"

The Tax information provides a view into his business career.

"Decades" modifies "tax information," it is not the subject.

The problem for understanding is that "of" makes "tax information" into an adverbial phrase and modifies "decades". So grammatically, it appears that "decades" is the subject.

No, "Decades of" is not an clearly not adverbial phrase. An adverbial phrase modifies a verb.

"Decades of" is a quantifier. It modifies "tax information", which is an uncountable noun. Uncountable nouns generally take the singular form.

I don't think you can put "decades" and "of" together like this.
"Decades" is a quantifier. "Decades of" is a wrong unit of grammar. "Of tax information" is different. It is clearly an adverbial phrase in which "of" means "about" or "concerning".

Wrong. If "of tax information" was an adverbial phrase, what would be the verb?

I don't exactly know what you are arguing Oristar. Your desire to change the word "of" to "about" seems ridiculous to me as a native English speaker. We don't do that in English.

In English grammar, there is correct and there is incorrect. We gave you the correct answer. Whether you are willing to learn it or not is up to you.

There is no argument here. This is not an adverbial phrase.

It is not incorrect to consider "decades" to be the subject instead of "tax information. If "decades" is the subject that "of tax information" is a prepositiobal phrase. In that case using the plural form of the verb "provide" would also be correct.

This wouldn't be grammatically wrong, but it would be a worse option. This is a sentence about tax information, not about decades