The almost exclusive usage of を in Japanese to mark the object of a sentence- is that the only specialized use of a letter or are there other languages where one letter is designated as being for a very restricted purpose?
3Depends on how you define ‘letter’. The ampersand (‘&’) was once considered part of the English alphabet, and that has a very restricted purpose. Similarly, # and £ both derive from abbreviated forms of libra (£ being just the L, # being lb), would you count those as letters? Å in English is virtually only used to refer to angstrom (ångström), but it’s not a letter normally used in English. Probably closest to the Japanese example: Greek uses letters of the alphabet for numbers, including some obsolete ones that are no longer used as letters, like ϡ (sampi), which now just means 900.– Janus Bahs JacquetAug 15, 2021 at 15:47
There are Grammatical signs in Egyptian Hieroglyphs that work pretty much like that Japanese character.– Sir CornflakesAug 15, 2021 at 20:48
は read as wa is also almost never used outside the topic marker in modern kana usage.– NardogAug 15, 2021 at 22:06
1@Nardog Except in the imperfective form of the copula used for negatives, では dewa.– Janus Bahs JacquetAug 16, 2021 at 1:06
1@JanusBahsJacquet I guess "the binding particle (係助詞)" would have been more accurate then.– NardogAug 16, 2021 at 1:14
First, a note: Japanese doesn't use an alphabet. It uses a combination of logograms, signs representing concepts (kanji) and a syllabary, signs representing syllables (kana).
However, having certain signs restricted to certain grammatical features isn't unheard of. In Hieroglyphic Egyptian, for example, there's a sign used only* for the dual marker, and in cuneiform Akkadian and Hittite, there's a sign used only for the conjunctions "and" and "or".
In English, we also have some symbols with very specialized meaning, like "&", "1", "#", and so on. Historically, "&" has sometimes been considered a letter of the alphabet, while the others generally aren't.
Finally, it's very common for logographic systems (like Japanese kanji) to have symbols used for very specific purposes, like a symbol used only for the word "festival". But since you're talking about Japanese, I'm assuming you know about these, and are only interested in symbols for grammatical function rather than semantic content.
* It's also occasionally used for a few other words that are pronounced as if they were dual, even if they aren't. I'd consider this a rebus usage, though, like writing the word "to" as "2".