You have it exactly backwards. Thou is singular and informal, ye is plural and / or singular formal.

Ye is a second-person, plural, personal pronoun (nominative), spelled in Old English as "ge". [0]

The word thou is a second-person singular pronoun in English. It is now largely archaic, having been replaced in most contexts by the word you. [1]

[0] https://en.m.wikipedia.org/wiki/Ye_(pronoun)

[1] https://en.m.wikipedia.org/wiki/Thou

Not sure how my comment is “exactly backwards” — it’s logically consistent with your (new) comment. My point was that, relative to you, ye makes a case distinction (nominative), and that relative to you, thou makes a number distinction (glossing over formal/informal).

We deprecated ye and thou, but it’s the latter that “requires” y’all, because only thou (relative to us still having you) distinguishes the number of people.

Edit: put another way, “thou/thee is the one with number” meant that reintroducing them would create a number distinction.