I left biglaw in 2014, at which time my firm had barely transitioned from Windows XP. This was because we used certain Word plugins/macros for formatting briefs, and they had not yet been updated to run on newer versions of Windows. As a result, we were all on XP way past its prime.
From talking with lawyer friends, I got the sense my experience was not uncommon.
Unless using Windows XP was somehow making them less money or costing more billable hours, why would they switch to Windows 7 or 10? I wouldn't even think about switching until an EOL notice went out. If it ain't broke...
From talking with lawyer friends, I got the sense my experience was not uncommon.