On the one hand, a sign like this definitely did have enough room for the full spelling of “through”. There seems to be no reason to abbreviate it.
On the other hand, isn’t drive-thru just, like, its own noun now? Part of me thinks this was always spelled correctly.
Right, the distinction I’m making is this isn’t just “normalized” but actually the correct spelling. As in, if a newspaper editor saw it written as “drive-through” they would be obliged to correct it.