DVT can lead to serious health issues, even death. In American Way (AA) Magazine, moving ones legs (especially on a flt. over 3 hrs) is important.
On (most) intl. flts, the economy cabin is more than twice as large as (F/BC). So a pax in F/BC, to get the proper amount of leg movement, would have to walk into Y--less essential for Y to go into F/BC.
On page 122 of American Way, Dec. 2015 issue: http://www.ink-live.com/emagazines/a...mber-2015/#122
they do suggest walking,