The Picard ManeuverYou can feel how cathartic this must have been for someone
xantoxisOn 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.
bleistift2According to Merriam Webster, “thru” is an acceptable, albeit less common, variant of “through”.
www.merriam-webster.com/dictionary/thru
mercDictionaries are descriptive, not prescriptive. They don’t decide if something is “acceptable”, just if it is widely used enough to report. If a mistake becomes common, it will enter the dictionary.