Firstly, I as a British English speaker from birth, I am not good at explaining grammar rules! So I will start from that point and stand to be corrected. 1. There is virtually never a comma before “that”, unless there is some other reason to use a comma, such as another non-essential subordinate clause ending there. (https://jakubmarian.com/comma-before-that-and-which/) In this case, I would suggest that 'universaly known' is a non-essential subordinate clause referring to 'truth', thus justifying the use of the comma. 2. That: relative pronoun. (https://dictionary.cambridge.org/grammar/british-grammar/pronouns/that) We use that to introduce defining relative clauses. We can use that instead of who, whom or which to refer to people, animals and things. That is more informal than who or which: She picked up the hairbrush that she had left on the bed. He was the first director of the National Science Foundation, and he funded science research with an annual budget that grew to 500 million dollars. As for appositive clauses ....I have to say, I really don't understand what they are!

The clause functions as a definition of "it." We can rephrase it several ways. It is a truth universally acknowledged, that a single man in possession of a good fortune must be in want of a wife. = That a single man in possession of a good fortune must be in want of a wife is a universally acknowledged truth. = The statement "a single man in possession of a good fortune must be in want of a wife" is a true statement.

Hi! just an extra thought as appositive clauses have haunted me a while. In the sentence you offered, I now think that "universally known" is a non-restricted appositive clause. It does not need to be set off with commas. However it acts as a hyperbaton, or figure of disorder, and thus disrupts the flow of the sentence. The comma is needed as a pause before the final clause.