How to segment old digitized newspapers into articles
As far as I know topic segmentation is not a particularly easy task with clean data, so it's likely to be challenging with noisy old French. 

It's not exactly the same problem so I'm not sure if this is useful but you might want to look into using stylistic features in order to help the model detect the changes between articles. There has been a fair amount of work on the task of style change detection as part of the PAN series (the task has run for 3 years, results and papers are available from the previous years).

Hope this helps.