How to deal with coauthors that have left academia
First of all, it doesn't matter where, how, or even if a person is employed.  Science can be done by anyone, anywhere, "academia" or not. Assuming the person has made significant contribution by the standards of your field, the only things that matter are:


Is it possible to contact them?
Do they want to be an author?


If they have contributed significantly and want to be an author, it is dishonest to not list them as an author.  If they've dropped out of research entirely, you may find yourself doing the writing work without their help, but they still should be an author.  If you can't contact them, err on the side of inclusion.

In fact, this is a place where I disagree with the letter of the Vancouver Protocol, which states that somebody can only be an author if they are significantly involved with preparation of the manuscript.  The spirit of the Vancouver Protocol is to prevent "gift authorship" and other unethical types of inclusion.  Imagine, however, writing an acknowledgement that says: "John Smith did all of the experimental work, but the long hours burned him out, so he left for a job at Netflix and we cut him out of the author list."  To me, at least, this feels like denying credit inappropriately.