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.

It seems like shorthand for signs that has been used enough that it’s basically normal now, like “lite” instead light, or “donut” instead of doughnut.

Right, the distinction I’m making is this isn’t just “normalized” but actually the correct spelling. As in, if a newspaper saw it written as “drive-through” they would be obliged to correct it.

The correct way would be “drive-through.”

“Drive-thru” is purposely spelled wrong to attract attention. The same as “Krispy Kreme” or “Dunkin’ Donuts.” It’s only “correct” in that it has become ubiquitous through usage.

Pretty sure thru is to save space.