The Old English letter later largely replaced by the digraph “th” was thorn, written þ or Þ. Thorn represented the dental fricative sounds heard in modern “thin” and “this,” and it was used alongside another letter, eth (ð), in Old English manuscripts.
Thorn ultimately derives from the runic character ᚦ in the Elder Futhark. It remained common through much of the Middle English period, but the two-letter spelling “th” became increasingly popular from the 14th century onward. As the shape of thorn grew less distinctive in handwriting, it could also be confused with other letters or abbreviations.
Printing accelerated its disappearance from English. Early printers often lacked a thorn character, and later texts sometimes substituted a visually similar “y,” which helped produce misleading spellings such as “ye” for “the” in modern reproductions of old signs.
Thorn did not vanish everywhere. It survives as a fully functioning letter in modern Icelandic, where it remains part of the alphabet. Yogh, ash, and wynn are other historical letters, but none is the canonical answer to this question.