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.
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.