> The use of quotation marks to indicate sarcasm or doubt is pretty new
I'm not sure that's true. A writer will always have some reason to use literal quotations instead of their own rendition of things. What makes you certain that skepticism is only a recent addition to the list of possible such reasons?
An extensive scope of reading material, most likely. If you like reading enough to read across multiple decades and centuries then one can usefully make such observations from one's personal corpus. This isn't something that can be easily automated as machine learning is not yet that good at the semantic level, but I concur with the grandparent poster that the ironic use of quotation marks is a relatively new journalistic trend.
I'm not sure that's true. A writer will always have some reason to use literal quotations instead of their own rendition of things. What makes you certain that skepticism is only a recent addition to the list of possible such reasons?