Over the centuries various sizes of paper have been made and popularized in different countries. In Great Britain there was a whole system called Imperial Measures ~ gallons ~ chains ~ ounces. Many of the Imperial units remain in use ~ although more carefully defined than being 'the area of a field that can be ploughed in a day'. Most countries now use the ISO216 and related systems , although manufacturing machinery and established traditions ~ and vocabulary '...take a length of two by one...' ~ take a long time to wear out.
Nowadays Imperial Size Paper refers to paper measuring 22" x 30" ~ 559mm x 762mm ~ with an aspect ratio of 1.3636.
What is special about this size? I surmise that Imperial is about as large as a handmade sheet as can conveniently be made by one person . Imperial sheets of paper are still made ~ and sold ~ for artist's use, although Half Imperial or Quarter Imperial are more commonly sold as convenient sizes for paintings. Machines designed to make that size of paper are still in existence ~ will they ever wear out? If artists wish to buy it ~ why not continue to make it? Most artists' materials are also available in ISO216 sizes.
An ISO A1 sheet measures 59cm wide ~ Imperial is 60cm wide ~
an ISO A1 sheet measures 84cm long ~ Imperial is 76cm long ~
an Imperial sheet is 8cm short of an A2 sheet.