What I wonder is, when did jabots appear.

They don't appear in The Highlanders of Scotland... the men are wearing ordinary white collared shirts and the ordinary neckties of the period.

I have a pile of mid-to-late 19th century photos of men in Highland Dress and I can't recall offhand seeing jabots.

But in my old Highland Dress catalogues from the 1930s jabots are seen worn with Prince Charlies, Montrose doublets, and Kenmore doublets (all, by the way, new jacket styles at that time). Perhaps jabots appeared at the same time as closed-collar Evening Dress jackets, to dress them up a bit.

(BTW jabots appear to be a modern attempt at capturing the look of the ruffles attached to the neck opening of many 18th century shirts, so in other words appear to be another 20th century quasi-Jacobean revival.)