Learn English – Is using “you” to refer to anyone, not the person you’re talking to, a known, specific grammar form

There is a concerted effort underway in the US and possibly elsewhere to eliminate sexism in language. In most books written more than twenty years ago, say, the masculine pronoun will be chosen.

The reader, when he encounters such an image, ......

For a while it was s/he.

Now one often finds she promoted to the place formerly occupied by (sexist) he. A sexist solution to sexism.

No.

Firstly, "thou"/"thee" is not modern English outside of dialectal usages (which I believe is its context in Uncle Tom's Cabin).

Secondly, you're right about the parallels to the two pronouns in Spanish, but "you" is the polite, formal option. Actually, Spanish is a bit more complicated, because it has (I understand) familiar and formal versions for both singular and plural. English is more similar to French; in both languages, the singular ("thou", "tu") is also the familiar, and you would use the plural ("you", "vous") to an individual if you wanted to be formal.

(Related reading: "T-V distinction".)

Thirdly, having said "no", let me now change it to "no, except when it is". Thanks to "thou" falling out of usage, most people are only familiar with it in old works, especially the King James Bible and Shakespeare's plays (both Early Modern English, from around 1600). The former, in particular, has caused "thou"/"thee" to be associated with formal situations, because God is addressed as "thee" (because it's singular, not because it's familiar).

However, that only applies to people affecting old-fashioned usage. They are quite likely to use "thou"/"thee" as a formal pronoun. People who use them because it's part of their dialect will, I'm sure, be using them with their original (singular and familiar) meaning.