surely everything here is now A sized (apart from SR print and other trade sizing), and in the USA more Letter format?

Foolscap paper is admittedly rare, but still in use in legal offices and the like (though not the most frequently used paper there either). It is still 'used', however, more often as a sizing template for files, folders, binders, wallets and the like. Look up any office stationery catalogue and marvel at the way the most common sizes for these are foolscap, not A4.