I have come across the mother of all "so's". Beowulf, an epic Old English poem of 3812 lines, begins, in Seamus Heaney's translation:
So. The Spear-Danes in days gone by and the kings who ruled them...
In the introduction, Heaney defends his use of "so" because it"
operates as an expression that obliterates all previous discourse and narrative, and at the same time functions as an exclamation calling for immediate attention."
To be fair, this was in the 1990s, before the "so" craze caught on.