The word "so" is correct and usable in written English. You can use it to show a connection between two ideas (similar to "therefore" or "thus") or to show a consequence of an action, feeling, or belief. Example: I stayed up late last night studying, so I'm feeling quite tired today.
And so it is.
And so they were.
And so it proved.
Quite so.
So it proved.
And so on.
And so forth.
That may be so.
Why so?
And so it does.
That is not so.
Ludwig does not simply clarify my doubts with English writing, it enlightens my writing with new possibilities
Simone Ivan Conte
Software Engineer at Adobe, UK