Learn English – What non-alphabetic characters are valid in English spelling

There have been plenty of proposals to extend or replace the English alphabet, including:

• Benjamin Franklin's phonetic alphabet
• Deseret alphabet
• Pitman's Initial Teaching Alphabet
• Interspel
• Romic alphabet
• Shavian alphabet
• Unifon

But plans? As Robusto pointed out, plans by whom? There's no Academy of the English Language.

It’s not really part of Modern English vocabulary. Depends how you define vocabulary. Does it include words and letters being used? Or does it cover only those words and letters that are officially in words such as a or apple?

Wikipedia says this about it:

Æ (miniscule: æ) is a grapheme formed from the letters a and e. Originally a ligature representing a Latin diphthong, it has been promoted to the full status of a letter in the alphabets of some languages, including Danish, Faroese, Norwegian, and Icelandic. As a letter of the Old English Latin alphabet, it was called æsc (“ash tree”) after the Anglo-Saxon futhorc rune ᚫ, which it transliterated; its traditional name in English is still ash /æʃ/.

It’s not a letter in the English alphabet. So depending on how you define vocabulary, Æ either could be or is not part of English.

Another strange letter would be œ, which used to be in the word diarrhoea when written as diarrhœa.