Starting sentences with "so" isn't a trend or a thing. However it may strike you, people aren't doing it any more frequently than they were 50 or 100 years ago.

So do sentences that start with "So do." Not every relative pronoun will work at the start of a sentence: You likely won't be able to get away with something like "He asked about the pocket watch.

Enter (I hope) the long sentence: the collection of clauses that is so many-chambered and lavish and abundant in tones and suggestions, that has so much room for near-contradiction and ambiguity ...