Everywhere I have ever seen it spelt, it has been written with the Latin letter. I have never seen it spelt as "arχiv" or "arΧiv", only ever as "arXiv".
I can understand why, for example, having a Greek letter in the URL would be undesirable, but if one is going to consider that letter to be Chi, then that should be the authoritative spelling and it should actually be spelt that way where possible.
Given the the
"Archive" homophone, it is obviously intended to be pronounced that way. The spelling is secondary to the common pronunciation, as usual
in English.
X is LATIN CAPITAL LETTER X
Χ is GREEK CAPITAL LETTER CHI
Everywhere I have ever seen it spelt, it has been written with the Latin letter. I have never seen it spelt as "arχiv" or "arΧiv", only ever as "arXiv".
I can understand why, for example, having a Greek letter in the URL would be undesirable, but if one is going to consider that letter to be Chi, then that should be the authoritative spelling and it should actually be spelt that way where possible.