The Collaborative International Dictionary of English v.0.48:

Yt \Yt\, Yt \Yt\ ([th][a^]t),
   An old method of printing that (AS. [thorn][ae]t, [eth][ae]t)
   the "y" taking the place of the old letter "thorn" ([thorn]).
   Cf. Ye, the.
   [1913 Webster]

The Free On-line Dictionary of Computing (30 December 2018):

yt

    The country code for Mayotte.

   (1999-01-27)