Its tough to pin down the exact date because of so many competing narratives during Palestinian identity formation but it was created mostly in the 20th century. Under the Ottomans local Arabs identified more closer as Muslim subjects within the Ottoman empire. During the 19th century anyone living in Palestine was equally called Palestinian.
There is decent article on Wikipedia called “Palestinian identity” that’s a brief overview but there are lots of academic works that say the same thing. As far as I know there is academic consensus on this point (you can easily look this up online).
I would also point out that although Muslims made up the majority of the region at the time the population was very sparse. For instance the population of Jerusalem in the early 1800s was less than 10,000.
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