It would be clearer to say that in the past "thou" was a familiar version, rather than an informal version.  It was used to show the closeness and intimacy of the our relationship with God.  At that time "you" was used with strangers, but it was not impolite or disrespectful to use "thou" with family and friends.

As Matthew said, however, the historical usage of these words is irrelevant. Now,"thou" is referred to as hieratic language, that is, a kind of formal language set apart for solemn and religious contexts.  It is not correct to call it archaic, since it is still in current use.  Nor is it "referencing old translations."

As we use it today, "thou" fosters a sense of the sacred because this is special language for God, while "you" is the everyday term.  I might not go as far as calling "you" disrespectful, but it is more appropriate to have special language.  Our worship is full of sacred things set apart for God.  The vestments are only for use in liturgy.  The Chalice is not used when one wants a drink of water.

We live in a culture greatly lacking in a sense of the sacred, so it makes sense that it is good to use words that foster it.

Anyway, my prayer book is from 1961 and has You and Yours all over the place in it - I never really paid much attention to it till now.

My second language is Spanish, I am 100% of Spanish blood, but I was educated totally in the USA. I speak Spanish fluently. In Spanish people are formally addressed as "usted" and close friends are addressed as "tu". My grandfather was a CEO of a very large corporation with over 3000 employees, and he was older in age than the owner of the business, and even after they worked together for 50 years, in the 80's they refereed to each other as Mr. and Usted.

The old Latin Mass missals in Spanish that I have all use the formal "Usted", and the modern ones went with Tu. There is a power behind that move in English and in Spanish.

Those that have eyes to see let them see.

But when "thou" was introduced, i.e when the first Catholic English translations were being made, "thou" was not a sacred or special term at all. It was the same word as was used for friends, children, and even animals. If we had a word with a similar meaning today that was substituted in, you can bet any money Last Tradhican would be calling it the height of disrespect and borderline blasphemy. To quote him "So we should talk to God informally, like we talk to nobodies?". At the time of the 1855 Missal, the more formal, solemn term would've been "you". So by his logic, he should have a greater problem with the 1855 Missal than with the new one.

At the time "thou" was first introduced, it signified the close, loving relationship we have with God.  And that was a good thing to do at time, in a culture in which this truth was obscured. (This is shortly before the rise of Jansenism, which saw God as very distant from humanity, to point where it developed into heresy.)

But that was a different time.  The language was different and spiritual needs were different.  We live in a culture that has lost the sense of the sacred.  The word that better addresses that need is "thou" not "you".  The connotations of these words in the past is irrelevant.

Today, "Thee" and "Thou" are formal, so it's how WE (in 2019) talk to God. "You" and "Your" is how we talk to the garbage man.

If they were reversed in 1600, that is completely irrelevant. We live in a culture that has lost the sense of the sacred.  The word that better addresses that need is "thou" not "you".  The connotations of these words in the past is irrelevant.