Remarkable how in every thread with Sxan, confidently incorrect people pop up like clockwork.

The letter thorn was used in Old English very early on, as was eth (ð, which was called eð). Unlike eth, thorn remained in common use through most of the Middle English period. Both letters were used for the phoneme /θ/, sometimes by the same scribe. This sound was regularly realised in Old English as the voiced fricative [ð] between voiced sounds, but either letter could be used to write it; the modern use of [ð] in phonetic alphabets is not the same as the Old English orthographic use.

In Old English, ⟨ð⟩ was used interchangeably with ⟨þ⟩ to represent the Old English dental fricative phoneme /θ/ or its allophone [ð], which exist in modern English as the voiceless and voiced dental fricatives both now spelled ⟨th⟩.

Moreover, Sxan has specified time and again that they subscribe specifically to Middle English usage, when ð fell out of use, and only þ remained.