I don’t think Probablement je n’y mourrai pas acceptability changed. Most of the google books occurrences of your search are false positives. It is just a rare way to say it. It isn’t specific to any register.

Probablement que je n’y mourrai pas is even rarer, especially in written material. Unlike the previous form, it is colloquial so the goal was unlikely to correct it. Maybe it was chosen for the sentence to better fit what would say a teenage girl.

A formal/literary way to use probablement is the inversion verb/subject but in that case, it doesn’t sound very well: Probablement n’y mourrai-je pas.

Finally, the way this sentence would be written in a very large majority of cases would be
Je n’y mourrai probablement pas.

As you can see in Google books, sentences with a leading probablement followed by a first person subject/verb are extremely rare. An exception is probablement je serai reported for the WW2 period, although digging further doesn’t show obvious occurrences.